School me, particularly on brands.  I see this Rangers 8piece kit on Amazon for $30 (plus 9 adaptor rings), but I'm damn suspicious of cheap photo gear.
Amazon Linky

Primary purpose is going to be for things like daytime lightning (want to do 3-5 second exposures to catch the bolts) and the typical flowing water stuff.

How many stops should I be looking at?  If I don't get a kit I'll probably get one for the Tokina 11-20 and the Nikon 24-120 lenses.

Honestly, I'd just eat it and get a good set of ND filters in a larger size and a set of step down rings.  The big difference is what the lens is made of, coatings, and most importantly how little color cast they give the photo.  Especially when you get up to your 'big stoppers' you'll start to get a deep green tint (it's almost like welding glass) so a photo with a grey card in it to set your post white balance is useful.

ND and ND grad are fairly inexpensive usually, but dear good a reverse grad ND is unholy expensive in comparison.  (dark in the center and tapering out both ways for sunsets and the like)

Of the most popular, (IMO) B+W have the least color shift but moderate vignetting, so YMMV.  You'll probably be wanting to get a variety honestly.  They differ on light conditions really.  The way I learned was to set focus, gaff tape the focus ring, reconfirm focus, flip the lens to manual, and then screw in the filter, but there may be a better way.

This. I never cheap out on filters. I use Nd filters a lot so I invested ina good set. B+W would be my go to for screw in filters. Lee if you want to go drop in. I prefer drop in as I can in a quick second remove everything and shoot normal. I had a few instances where screw ins would get stuck and with rapidly changing light conditions every second counts.

Of course drop ins are more money then screw ins as you have to buy the adapter then the filters on top of it.

The only reason I don't have drop in filters yet, is because I'd need to drop $1k on the set I'd need.  

I've spent a bit of money on ND filters and I offer the benefit of my experience.  

You are correct to suspect low-cost options and my experience largely supports that. I currently use square filters in a 2-filter holder and I can say that the Haida 150mm filters are very good for the price. I had the 3- and 6-stop versions and they both were close the advertised density with the greatest variance being a bit more than a 1/2 stop on the 6-stop filter (making it effectively a 5.5-stop filter). Color cast was very manageable and far, far less than LEE and Tiffen filters I had.

I currently use Breatkthrough Technology filters. These are premium filters and both my 3- and 6-stop filters are within 1/3-stop of advertised density. Color cast is negligible. My structured, but un-scientific test method was: 1) Set up an X-Rite ColorChecker Passport Photo in an indoor scene with daylight balanced constant lighting; 2) Use LiveView and take a photo without filters; 3) Install the 3-stop filter; 4) Use LiveView to adjust exposure and take a photo; 5) Remove the filter; 6) Install the 6-stop filter and take a photo; 7) Import all photos into Lightroom and use the white balance eyedropper to confirm color cast differences between the three. Repeat the series with other brands of filters.

[My opinion:]
I'll be sticking with the Breakthroughs as they are the closest to advertised density and have the least color cast and vignetting - and that's what one would expect after paying their price.

That said, the Haida filters offer very good performance for the money. I didn't keep the Haida filters because the Breakthroughs do outperform them (though by a competitive margin). The only reason I bought the Haidas at all was I had a gift card discount and wanted good filters for a trip I took while waiting for the Breakthroughs on backorder. (Breakthrough took a few months between taking my order and releasing their 150mm option.) I'd seen an online review of the filters and rolled the dice on them. If I hadn't already paid for the backordered Breakthroughs, I'd probably still be using them.

I will not willingly buy another LEE ND filter. Despite their solid reputation in the industry, their color cast is just too far off (especially on their 10-stop Big Stopper) and the difference between advertised and measured density was greater than Haida and Breakthrough.

Nor am I excited about buying Tiffen, Formatt Hitech, or B+W NDs again. I've already shed my tears down that trail.

In my experience, a combination of a 3- and a 6-stop filter covers all the ND scenarios I've encountered to date. If 6-stops isn't quite enough, adding the 3-stop to it has done the trick for me. I've found greater flexibility with the 3-/6-stop combination in my kit than my old combination of LEE Little/Big/Super Stopper filters. FWIW, I found the LEE Super Stopper forced longer shutter speeds than I really wanted to use and the "creamy water" effect is almost always sufficient for my needs at 9-stops of ND (making the Big Stopper way overkill).

My current rig is a Haida 150mm holder with 2 filter slots and my widest lens is a Sigma 12-24mm f/4 ART. The holder does present a vignette on the sides between 12 and 14mm on that lens. My options are re-compose, crop, or zoom to at least 15mm (which is another form of re-composing the shot). This vignetting is not an issue on the Nikon 14-24mm (according to other users), so it's a combination of FOV deltas between the lenses and depth of the curves on the lens hoods on which the adapter sits. I still prefer the Haida filter holder primarily because it is the only 150mm holder that I found on the market that will fit my Sigma lens while mounted to the camera (a rare feature in the market). It also allows me to lock the filters into position (whereas the LEE holder will always allow the filters to rotate). Locking the filters in position may not seem like a big deal, but I've accidentally bumped the filters a few times after positioning the CPL. You (and other readers) may be careful enough that this isn't a consideration.  

Why did I move from screw-ons to squares? That would be my phase of using graduated NDs. The square format allows positioning of the ND grad horizon independently of the composition whereas a screw-on ND grad has a fixed horizon forcing one to position the landscape horizon exactly in the center of the composition. After moving to squares, I gradually stopped using NG grads - especially after I learned the technique of exposure blending in post-processing. Additionally, I acquired the Sigma 12-24 in that timeframe and that lens will not accept screw-on filters. I stayed with squares because I like the ability to compose my shot and set my focus before quickly and easily mounting all my filters for the shot in one quick motion. Screw-ons aren't as convenient in that regard, but they also don't require a separate holder that takes up space in your camera bag. This serves as a reminder that life abounds with the need for compromise.

To minimize the number of filters I carry (i.e., keeping squares versus adding screw-ons to the collection), I kept the Haida filter holder and bought a screw-on adapter that lets me mount the holder on my Sigma, my Nikon 70-200 (w/77mm thread), and my Nikon 24-70 (w/82mm thread). My total filter count on my hikes is four: one square 3-stop ND, one square 6-stop ND, one square CPL, and one additional screw-on CPL (for general walk-around use).

I remain on the lookout for a better filter pouch, but I've not found one better than the Haida pouch I have. The annoyance I have with the pouch is it doesn't mount very well on my current camera bag. I can attach it on the side of my bag by running one of the bag's straps through the pouch's belt loop and I can also run the strap (barely, though) through a D-ring on the pouch (intended for a shoulder strap). However, this is a sub-optimal solution and still leaves the pouch swinging way more than I'd like. Therefore, I continue my search for a better pouch.

If you're looking for lightning shots, I recommend using a lightning trigger (like this one) versus relying on an ND. They can be had for the price of a good filter and you'll walk away with a far higher percentage of keepers.

I am no expert, but I like playing with my one Hoya 10 stop screw on. (52mm)
Fountain by Chris Barzyz, on Flickr

Fountain by Chris Barzyz, on Flickr

Wow.  Just another case of not knowing what I don't know.

Thanks for the pointers, off to do some more research.

Another vote for Breakthrough Photography ND filters. Attached File

Damn that's a fine pic ! where is that ?

Look closely at the tree trunk...  I think you'll find that is a very unique tree indigenous to central Florida.