Re: horse power
get a 21' deep V center console, suchas a Sea Hunt, Sea Chaser, etc. You want room to move around. Avoid the heavily padded sofa boats. Way too crowded. Standard rig on the 21 of that type is a 150; maybe bump yours up.
Anyway, focus on the boat first. If you are buying used, you will probably get it with a motor already on; try it out and if you like the hull, buy it and upgrade the motor later (which is not all that easy unless you buy a new one, the cost of which will astound you.)
One factor is the boat weight and design. I push a 19' Carolina Skiff, flat bottom (totally flat) with a 50, heavily loaded.
Think about how much skiing you'll be doing; the bigger boats can be used for it but it eats up the gas and is not as manueverable.