The fore stock on the model 500 is usually fairly noisy and the old ones only have a single bar on one side of the pump. Might check that out on the purchase gun's in question.
For 365 dollars a lot of 870 wing-masters are available used and everyone (including mossberg) makes barrels and choke tubes for them. Rock solid, inexpensive, unable to be shot out, the 870 has it's advantages. Look at this 125 dollar version at gun's America.
http://www.gunsamerica.com/976924922...Wingmaster.htm
That would leave about 240 dollars left over to buy aftermarket barrels and choke tubes and a good cantilever fully rifled slug barrel. like this one.
http://www.midwayusa.com/eproductpag...campaign=10636