Brian- any cheapy AEG will do fine, something small like an mp5k or uzi would be handy (easily concealed and less obvious to anyone watching). I'd suggest a quickl look through the retailer lists on Arnies airsoft. Airsoft guns as such sting when you are hit, but its rare that they will break the skin- only at very very close range and/or with a highly upgraded gun. Failing that; Blackwidow...
Rivermaxx- I would look for something like a Daisy repeater rifle or something similar, plenty of cheap airguns out there for backyard fun.