People have 55g tanks fully setup for FW here for about $100 on craigslist. It might be more expensive to buy all the sw stuff, get rid of it (maybe $ell it?), and replace it w/ fw equip than to buy used, or maybe even to buy new.
Depending on the cost, I would probably just let the sw people take advantage of the deal, and find a fw deal. Or maybe you would be interested in sw?
But unless in was practically free, you would still buy the sw setup for however much, then fw filter, then fw lighting (optional, I guess), etc. The filter itself would probably cost you $40 new, if you just wanted a basic power filter.