Uppers are easy. Get receiver, bolt carrier group, barrel, barrel nut (comes with most free floats I think), free float, gas block, and gas tube. Muzzle break as well if barrel is threaded an it’s going on post ban lower.
In NYC, all firearms need a permit, so make sure if someone from there is interested, that they can legally own one.
There are no assault weapons in NYS. Or any state for that matter. NYS just has its head up it’s ass and considers any semi-auto that has two or more of the following to be an “assault weapon”: pistol grip, detachable magazine, bayonet lug, flash suppressor, telescoping/folding stock, threaded muzzle capable of accepting a flash or sound suppressor, and I think that’s it off the top of my head. Most ARs, AKss and such have the pistol grip and detachable mags so you can’t have anything else on the list unless the lower receiver (or part with the gun’s serial number) was made before September 1994. Also, no semi-auto can have a magazine that holds more than 10 rounds of ammo unless the magazine itself was made before 9/94.
Semi auto shotguns also can be considered an “assault weapon” if they have a pistol grip and can hold more than 5 shells. So you’d have to pick one or the other.