I'll echo the suggestion to start with the canonical 5.56/.223 AR.
You should understand that you'll need several tools if you intend on building the AR up from pins and stripped upper/lower receiver(s).
You'll need the barrel nut wrench, a torque wrench, the spanner for the buffer tube, roll pin punches (get a set), a receiver block to hold the upper in a vise, a screwdriver or hex driver for the pistol grip screw, etc.
If you're going to put on the barrel extension, you'll need some more tooling to do that. First time out of the gate, it's easier to buy a barrel pre-contoured with the barrel extension already on their and the gas hole pre-drilled. You probably want to buy your bolt carrier with the gas key pre-staked on.
Total cost of the tooling (assuming you have a good shop vise already and some small ball pein hammers) will be about $200+.
You really might want to watch a video, get hands-on instruction or at least get an armorer's manual for the AR before you begin. There's very little that isn't straightforward on the AR, but there are some things that aren't immediately obvious unless you've built one up from the pins. There are a couple of points where you can break stuff off if you ham-fist the driving of pins into the lower - particularly the "ears" of the receiver that hold the bolt catch onto the lower. Getting the dust cover spring installed correctly can frustrate the first-timer as well.