As you seem to like the rotomoulded polyethylene boats, the only one that has not been mentioned so far is the Escape Playcat. These can be picked up brand new very cheap, £3999 in the UK.

However I think they may be cheap because they are no longer being made, I heard they were in trouble and a quick look on the Escape website revealed they only seem to be selling monohulls at present.
http://www.jowatercraft.com/Sail.html

Another option, though not a cat, could be the windrider trimarans, I bet the rave would be a hoot on a lake.
http://www.windrider.com/wrproducts.shtml

Finnish dealer http://www.windrider.com/dealers_int_popup.php?brand=windrider&country=finland



Gareth