Well, I can only give you my experience, but I think 1.5HP will probably suit you fine, unless you're wanting this to be doing a lot of resawing of dry wood. I own a 14" Ridgid with a riser block, and upgraded the base and table to 24x24", and swapped out the 3/4 wimpy made in china motor for a single phase 2HP Baldor motor. I have 12 3/4" under the guides. The only time I had issues was when I was cutting a 12" thick piece of live oak, notoriously difficult to cut. Even with a new blade it bound a bit and made burn marks. I now use 2 tpi. blades from Suffolk, and have had no problems with it bogging down the slightest bit. The only time I've stopped the blade the belt slipped. I've never tried to resaw dry lumber over 8" thick, so I can't comment on motor strength. Honestly the blade sharpness and amount of tooth set has been more important than power when cutting green wood. I just wish I had more height under the guides!
Edit: I realized I didn't answer all your questions. I did have a 1.5HP motor for a time, and when wired 110V, it would bog down, but it was more of a voltage drop problem. When I wired it 220V, it worked perfectly and had every bit as much power as the 2HP motor. If you upgrade, I'd bet 3HP would be more than enough. I don't know what the powercraft 20" capacity is, but unless you're cutting over 12" thick stock, you probably won't use 5HP. Just my humble opinion. Again, blade sharpness has always been more important with my experience. A sharp blade is also safer; less wandering in the cut, and less pressure needed to push the wood through the saw.
