That 870 police trade-in at Buds is a great deal. I just bought an 870 police trade-in at my LGS for $175, but it is MUCH more "well loved" than the ones at Buds. You could get the one from Bud's, add a +3 and still be within your budget. If you ARE going to be adding a mag extension, either go with a Remington factory one or another steel extension. Stay away from the polymer ones...
Silencers have NEVER been illegal !