Rear sight: Magpul MBUS is good and inexpensive, YHM, Midwest Industries and Troy all have very good rear sights. Troy is very expensive.
It can be if you get the models will all the bells and whistles. For use as a primary sight or a backup to co-witness with a non-magnified optic, they make some fixed models that are quite affordable including some with tritium inserts that go for $110.
Nikon African Monarch 1-4x The Nikon is very clear, great eye relief. Both hold zero. Other good 1-4x scopes: Burris Tac30 or MTAC, Nikon M223.
I'll second the Monarch African. I have the 1.1-4 with the larger tube and objective rather than the 1" version with the 20mm objective. I'll also add the Bushnell Elite 1.25-4 to the list. There are a lot of good optics in that magnification range priced from around $200 to several car payments.
Scope mounts: Primary Arms carries their brand, Burris, ADM and nikon mounts. All are good. The Primary Arms is the cheapest and the ADM is the most expensive. I had a Burris PEPR. I should have kept it. It is a nice mount. The ADM is my favorite. It is rock solid.
I have several ADM mounts and they are very nice. All ADM mounts are quick detach. Decide if you need that feature before you go mount shopping. Most people, if they were really honest with themselves, don't need a QD scope mount on their AR but "have to have it" because it's the tacticool thing to have, just like backup sights. Being honest about your intended use for your AR and basing your purchase around that can and will save you a lot of money and leave you with a rifle better suited to what you want it for in the end. I got a flyer the other day from Nosler with a bunch of ammo that I ordered that made me laugh in this regard. It was for the new Nosler/Noveske Varmageddon AR intended as a highly accurate varmint rifle with a quality barrel. The rifle comes with a Magpul PRS stock (I have one on my dedicated varmint AR as well but mine is a bull barreled 24" beast meant to be carried from the truck to the bench to shoot p-dogs not a "carry rifle" with a medium weight 18" barrel) a top-railed rifle length forearm (no real problem there but a basic float tube would have been plenty and run $150 or so less at least). I can deal with that but the real kicker was the inclusion of big $$$ folding irons. On a rifle meant to be run with magnified optics for varmint shooting?!? Then you look at their "package deal" that adds optics and a mount into the mix and it bring in a rather pricey Leupold (whatever, they're all rather pricey) and a fixed(!!!) scope mount. Kept those backup though, which now are completely useless in addition to being pointless.
If you want a rifle to shoot at the range, don't bother with all the crap hanging off of it. Get a good barrel, a basic free-float forearm (no rails) a good optic and a solid fixed mount and spend the rest on ammo. Same for a varmint or hunting rifle.
If you want a rifle for self-defense, you can really forgo the quad-rail there as well, since the only thing you *might* put out there is a light and possibly a forward grip or handstop, either/both of which can be mounted on normal hanguards or on short segments of rail on a basic float tube. All the extra rail space makes for a more expensive forearm that is bulkier and more likely to snag on things. From there a non-magnified, rugged optic (EO Tech and Aimpoint are the only things I'd look at here) a mil-spec front sight and a fixed rear sight would be all I would bother with. No frills, light weight, simple and reliable.
