Stumbled across this video yesterday that gives a good breakdown of setting up a good rifle in your price range but maybe on the higher side. Nothing wrong with buying optics, lights, and slings second hand off the for sale section here, tacswap, or ar15.com EE. They don’t mention much about having ammo until the end but if you are waiting to buy until your roommates are out, use that time to save $ for the rifle and then buy the ammo/mags in the meantime.
Regarding the roommate situation, that would definitely suck. In college my best friend and I are big into guns, we turned out other roommate who’d never shot before into a guy who loves the Ruger 22 series of pistols cause of how simple it was to shoot.
For security purposes, ever thought of having a rental/storage unit? Few of us did this in college and when I was in the military to store either stuff we weren’t suppose to keep in our living quarters (guns) and to just keep extra stuff we might need but didn’t want year round in the room with us just to save space. Just find a good one with security cameras and buy a quality unit lock. Heck, some even have climate control units (best friend owns Southside Storage in Mebane) which is where I learned about some of the storage game