A 16" mid-length profile AR-15 is what you'll want, unless you want a bolt action. TBH, I would recommend an AR-22 if you don't plan on shooting people or aliens with it. The cleaning is slightly different than a full fledged ar15, but it's cheap, more fun to shoot (imo), and will teach you much better fundamentals than starting with a larger caliber.
Joebobsoutfitters, primaryarms, and riflegear are all very good online vendors. Palmettoarmory has the absolute best prices but there shipping department sucks, even though they will right any wrongs on their part.
So hypothetically, using a 22 would be better for training proper fundamentals in a group of newly interested gun owners?