PoJ, I have a 2003 21' with a 200 Merc EFI. I don't think you would want any less HP. I think a 150 would be underpowered. There is a charter captain in Morehead City, NC who put a 200 Suzuki on his 21 recently and seemed to be extremely pleased. His boat is Old Yeller and I think his name is Danny Adams. If you could talk with him he could probably give you some insight. Thought this might help. Good luck.Here is his web site.
http://oldyellerguideservice.com/captain.htm