I work at a used game store and we clean/test/price consoles. Surprisingly enough the best model (IMO) is the 360 S 250gb HDMI. It ends up going for about $90 CAD with cords and controllers.

If the system is an Acrade/Elite I highly recommend you do not buy it as it has a terrible shelf life. If it works one day it won't work the next. Super unpredictable.

Buying from a friend I recommend you test it thoroughly and if you buy it to clean out the inside, that shit gets really nasty. Also buying a new AC cord can up the performance a lot as well. Good luck