I built my AR as a full-length HBAR with a removable carry handle and fixed A2 stock. It has a fixed-base (non removable) front sight of the correct height to use with the A-3 carrying handle's sights. I want to mount some sort of optic on the receiver rail, with a flip-up sight as backup (I am leaning towards a red dot with a flip-to-side magnifier; my current optic is an Israeli SUIT with rejuvenated tritium illumination; I have both a carry-handle mount for it as well as a rail mount). First off, is the configuration I am considering even feasible, or would the magnifier interfere with mounting the BUIS; second, are there any BUI rear sights available that are compatible with my front sight? Or would I need to replace my entire gas block and install a flip-up front sight as well?

You should be fine with any of the standard folding rear sights (not micro)

Thanks! That opens up a lot of possibilities.

Now, here's another question: does a red-dot sight HAVE to be an Eotech or an Aimpoint to be decent? Or are there any out there for ~$100 or less that aren't airsoft-grade toys? Not looking for a mil-spec, combat-grade optic (that is what the S.U.I.T. is for, if I need it), but I DO want something that will hold zero and stand up to an average day' out in the fields and woods hunting, and while the Aimpoint and Eotech are great, they are also EXPENSIVE - I paid less to build my AR than an Eotech or Aimpoint costs, on average!

While not under $100, take a look at the Primary Arms and Vortex products

In that budget, the Bushnell TRS-25 is probably your best bet. For just under $200 budget your options really open up with Vortex and Primary Arms.

Also take a look at Holosun. You'd have to stretch that budget by about $40 to get into the 503 series (circle-dot), but they're easily worth it. I have a few 503C's that I like, but I'd not buy them again. I'd rather just have the 503GU (no solar regulation, exterior battery turret that's easy to change battery, and shielded turret caps. Also has auto on/off motion sensor, unlike the C series).
The 503C is kind of joke. The "C" denotes Solar "regulated" (no, it will NOT run on solar power alone, it REQUIRES a functioning battery. The "solar" mode simply modulates brightness based on ambient light). It doesn't really extend the battery life much, and you have to manually turn on/off the unit. It's a great RDS, but IMHO, the 503GU is the way to go (simpler, easy battery changed, no need to turn on/off, etc).

Okay, two MORE questions (thanks for bearing with me...)

1) Are the Magpul MBUS (Gen2) flip-up sights any good? I know Magpul is supposed to be a good brand of magazines, but I have heard their sights are plastic.

2) As far as magnifiers for dot sights go, do they have to be specific for use with the red dot I go with? Or are they pretty much generic as long as the magnifier is at the correct level to work with the red dot? The Bushnell T-25 looks good, but the Sightmark Ultra Shot seems to have gotten some good reviews as well.

1.  The Gen2 Magpul MBUS sights are very durable, some might say even more durable than their metal competitors.  They are absolutely good to go.  The only negatives I've found is they are larger and won't fit under certain scope combinations, and switching between the large and small aperture isn't as easy as some.  But they are really good sights.

Magpul also makes a metal version that is really well made and has a tiny footprint called the MBUS Pro.  I'm actually using the 45 degree offset variation on my rifle now.

2.  While not as well versed with magnifiers in general, I think you have it pretty well correct.  Most seem to be universal and will work with a variety of red dot options.  Hopefully someone will come along to confirm or deny this.

You can find Aimpoints for $350

IMHO, the Mbus PRO's are worth the extra. I'd check them out if I were you.

Which is about what I paid for my AR build. Only thing is, I spread the cost out over time (started with an 80% lower). Heck, I only spent about $60 for my SUIT, including a new tritium vial (lucked out on that one!).

Holy crap alright didn't know it could be done for $350

I got REALLY lucky as far as sales on parts went, plus I had a few parts hanging around from many years ago, left over from another build I had done and had upgraded before I traded it away for an AK (which I then sold during my college years due to a desperate lack of cash). Glad to have an AR again, though another cheap build might be tricky!

Got a SWEET deal on a Magpul MBUS PRO on EBay. It was sold for $14 with a broken clamp - and Magpul sent me a replacement clamp free of charge. Now sitting comfortably on my AR!