Right then, let's get you squared away.
First off I'm not sure what plates you were looking at but there are plenty of sub $500 ceramics out there, which brings me to my first question. Exactly what plates do you want? You said III+
or IV. These are not the same and even within the same protection level there are differences. I will explain further down below.
Second, if you could lay out a firm price range that will help a lot.
Third, what is a "sketchy brand"? Yes, there are less than reputable dealers/manufacturers out there but I need names. You said you've never "delved into body armor" and based on the rest of your post I'm guessing you have little, if any experience with it? It may be that you were looking at legit dealers/manufacturers but didn't know, but you very well could have ran into some shady stuff. There is a lot of cheap chinese armor hitting the market now that likes to masquerade as something it's not, so you're apprehension is understandable and IMHO smart. Especially when dealing with items you may depend on to save your life.
---------------------
Now let me lay down some general info...
Yes, some dealers require proof of citizenship/residence/etc. Some dealers are also Mil./LEO only. Just the way it is. SKD Tactical for example has a body armor policy that requires you to send in something (can't remember what at the moment, but something like drivers license/birth cert. etc...) when you buy armor from them. I disagree with their policy and hence don't buy armor from them or anyone else with such a policy. This is a personal choice, and what you choose to do and who you choose to deal with is up to you.
As for a carrier I find this is a highly personal choice and everyone has their preferences. What works for you with your specific setup, purpose of use, etc. may be totally different from someone else's. However I have heard good things about all 3 carriers you mentioned. Something to note to is size your carrier with your plates. If you buy a size large carrier, get large plates. Otherwise they may not fit or the fit will be sloppy.
---------------------
Now I'm going to explain some stuff about plates and protection levels. Ready? This'll be a long one...
First of all level III+ is not a proper NIJ rating. It seems to have come about with steel plates and "special threat" plates of varying materials. What it usually means is a plate that meets level III standards while also defeating "special threat" rounds, usually stuff like M193. Where a level III steel plate will stop lead .308 and lower velocity 5.56 it may not stop M193. That's where the III
+ comes in. Usually plates marked III+ will stop such rounds as M193 while not meeting level IV standards, meaning it can't stop .30-06 AP and the like.
That's where level IV comes in. Level IV armor is designed to stop AP rounds, including .30-06 black tip at high velocities. Level IV armor is the highest actual armor rating attainable.
Now lets muddy the water some. There are some plates that only meet their full protection rating when used in combination with (otherwise known as ICW) soft armor backers, usually IIIa rated. These are
usually level IV ceramic plates that were made without the backing material behind the ceramic strike face built into the plate. The soft armor backer acts in the place of this backing material. ICW plates are usually lighter and thinner, but you MUST wear the appropriate soft armor backer behind the plate for it to meet its full threat level. Plates that do
not require a soft armor backer are called "stand alone" plates.
To clarify, an ICW plate rated level IV will require soft armor (almost always IIIa rated) behind the plate for it to reach its threat level of IV. Without the soft armor it may stop bullets but blunt truama and back-face deformation will be extreme, plus other hazards. A "stand alone" level IV plate does
not require the wearing of any soft armor behind it. It will perform as intended by itself.
---------------------
Now, as for places to buy plates the first one to come to mind is HighCom Security who are actually offering 20% off right now. The coupon code is available through the the news headline right here on Arf near the top of the page under "Site Notices". You're welcome.
There are others besides HighCom but I'm less familiar with them and I'm pretty sure you can find exactly what you're looking for there anyway. I personally don't know the ins and outs of all their plates but others here do and someone else will hopefully be along to help out.
---------------------
Sorry if I missed anything but it's late and my eyes are trying to shut even as I type this. I'll be back tomorrow though to help you out more. Please do try to answer my first three questions, that will go a long way toward helping you out. And I know I laid out a lot here but if you can try to narrow down exactly what it is you're after that will also help. Hopefully this info will help with that.