As far as the riser height, it depends on which cowitness you want, bottom 1/3 or absolute. There are a couple threads over the last week or so that explain the difference, with diagrams. The $10 UTG riser at Amazon does very well, the medium height (3/4") gives absolute and the tall (1") gives bottom 1/3. I use the tall one.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek