1. I don't miss having one.
2. If you are going to get into competitive shooting at long ranges, a scope will be a necessity. If you intend to get old, a scope will be a necessity (old eyes don't focus well on the close in iron sights). If you get this wrong, you buy a new upper, and go on your merry way. You'll want more anyway.
3. I use a black padded sling in the M-60 style. Longer than you need, so you can shorten it, and place the padded section where desired. Cheap, too. Not necessarily the fanciest.
4. I use aluminum military mags, both 20 and 30 capacity. There is a magazine FAQ floating around. Find it and read it. Good stuff in there.
5. I use a set of double insulated M4 style handguards. I like them. Feel good, look good. YMMV.
6. My AR has the A2 style butt. It feels better to me than the pinned fake adjusto-stock. It gives storage, a place to store the sight adjuster tool, pin pushers, even a cleaning kit. It gets my face to the right height for the sights. Again, YMMV.