The EBB would prolly be best for indoors im not sure on the shotgun. Mini Electrics don't have a lot of range but I don't know how big your indoor setting will be.
Well your best bet for a cheap shotgun would be the UTG 870. I think that would be your best bet over all by far unless you really desire a full auto gun. It can be bought with free shipping at www.airsoftxf.com
I have the Taurus and the mini they sell at Wal-Mart. both are good. I would go with a Wingmaster instead of a Stinger 30 if you have a G.I. Joes around close.