You don't say what your intended purpose is for this rifle, we can only assume it's a general intro to the AR world not a "snipe prairie dogs at 300 yards" rifle or "one rifle to get me through the zombie apacolypse" rifle. So in that case it's tough to beat the upper/lower kits that PSA sells. When you start trying to piece together an upper it can get expensive very fast. In your shoes my suggestion would be a 16" barrel mid-length gas upper that PSA will probably have on sale soon (with free float rail).16" lets you swap out the muzzle device easily in the future, mid-length gas is generally considered easier on the BCG. Free float rail gives you better accuracy and easy attachment of doodads. Pair this with one of their lowers or lower kits and you can probably spend around $600 or less (not including optic). The upgrade to the trigger is also a good idea but not immediately necessary.
They are fun guns to tinker with, have fun.