For target shooting I'd also get a 9mm for the cheaper ammo and lighter recoil. (In fact I have 7 of them.) But 45ACP is a great caliber if that's what you really want. Nothing wrong with a Glock in .40S&W either, all three of my Glocks are .40s.
I have 64 year old eyes and like the TFO fiber optic sights on a Glock. You might take a look at them.
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