I second the 1/4 hp recommendation, anything less may not bring temps down low enough
I've seen used chillers show up here time to time, some old, some new. but IMO this is one of those items that you might be better off getting brand new, like everything in life exceptions do exist.
On a side note,
I need a three story house, and new sports car, but I what I think he's look for is advice on purchasing a chiller for his 90 gallon, hahaha