I am exploring possibilties on an AR with a 20" barrel chambered in .308/7.62 Nato (functionality and commonality so please dont argue). I dont have a lot to spend but dont want it unless its a reliable firearm. I am toying with the idea of building one so I know the ins and outs if something malfunctions, breaks or what have you and can fix it myself. It needs to be able to accept 10 round and 20+ round mags. I want iron sights and scope possibly also an undermount tactical light as it will be a multi-purpose addition to my meager arsenal.
What can I expect to be the "most critical" purchase as far as effectiveness/reliability is concerned? The most expensive? Stock...stripped lower...parts kit...upper. Also, what scope should I add for as close as 100 and far as 500 yards? 3-9x40, 4-12x40, other? The purpose(s) will be deer hunting, range practice/fun and possibly some long distance 500-600yd shooting.