If you’re sold on the AR platform, but want to build a custom rifle yourself to your requirements, get a stripped lower receiver $150 or so and get an upper kit already headspaced for 5.56 NATO as opposed to .223. Check Shotgun News- there are many upper suppliers.
If you just want an AR, buy a used one in good shape; they’ve come way down like you say and you can add any gee gaw from there. The tool requirements are minimal.
If you’re not stuck on AR, and your anticipated need is close range, dead nut reliable you might consider the AK as a platform, or even the Romanian PSL (7.62 x 54R) as a longer range .308 equivalent (in firepower). Going Soviet gets you a lot more firepower and ammo(and practice) for the same money.
I own 2 AR-15s, one of which I built myself, but I still do not understand the difference between .223 and 5.56 NATO. I thought they were, for our purposes, exactly the same.