Being a monohul sailor my whole life, I am not totaly comfortable with what to look for in a used catamaran. I am looking now and like the Lagoon, Leopard - the south african boats have more appealing lines.
If you are looking at used boats you might take a look at the merits of galley up vs. down and wheel position, often either aft or on the bulkhead, two big issues when looking at a Cat. Under deck bridge clearance can affect the ride and hull volume from builder to builder. Lots of good choices though!!