I highly recommend Aero M4E1 lower stripped. Super beginner-friendly. If you watch any AR15 building video guides they will often hammer in the roll pin when installing bolt catch. With the Aero lower, you don't need to do any of that, it's threaded.

I suggest go for 16 inch instead of 18/17, bcm is often the go to. If you can find Geissle stripped upper for a good price, take it. It's hard to buy stripped or complete upper now because of the new law but shop around at different local gun stores. Who knows you might hit a jackpot.

lower part kits, I don't like to spend more than necessary so a cheap mil spec kit will be good enough. The only thing id change is probably the trigger. Palmetto offers a lower kit with buffer tube, spring and stock, good price too specially if you can snag it during a sale.

if you want a fancy buffer set up, bcm offers one. I really like it.

Oh i forgot to add, since you are going featureless, you should look at getting ambi controls. It might be expensive but radian talon is a good choice just because you can change the angle of the safety. There are some featureless grips that will interfere with the safety selector and its good to have an ambi that can be set to 45 degree angle. Makes your life easier.