TexasCHLforum.com

They're very easy to work on - don't be intimidated by the sheer volume of info out there. If you can buy a complete rifle, I would recommend that - it's often said that frankenrifles tend to be the least reliable ARs (probably due to tolerance differences between upper/lower batches from different manufacturers?). The most common caliber for these rifles is 5.56x45mm - by far. Judging by your criteria, this is probably the only caliber you ought to be looking at.

A 1:7 barrel is preferable - most surplus ammo out there is going to be M193 (55 grain) or M855 (62 grain), and the 1:7 twist was specifically chosen by Uncle Sam for these loads. The faster 1:7 twist will allow you to stabilize the heavier "modern" loads (usually 68 grain or heavier) that are most suitable for defense and hunting (although they are expensive). A slower 1:9 twist is much more common with civilian rifles, but less desirable unless you intend to primarily shoot light varmit loads (<50 grain).