short barrels are best for slugs. 18-20" is best. After that point, the slug has reached it's maximum speed, and now it's just slowing down in the barrel due to friction.
An 870 with a rifled barrel shouldn't be more than like $400. My brother-in-law picked up an 870 with both barrels for just over $400 if I remember right.
here's a pretty good article on slugs and slug guns:
http://www.gameandfishmag.com/huntin.../gf_aa106503a/