I don't know about hp requirements, I'd assume it's more about the rigidity of the machine to withstand the tool pressure without deflecting.

Can you torch your workpiece as you spin it? That may significantly reduce the hp and ridgity needs of your shape you want to spin.

Also an option is to just buy a spinning lathe, the spinning lathes I have seen in person weren't very big but stupidly heavy, last one at auction I saw looked to be only about 15" swing maybe 7 feet long but was easily pushing 8000lbs.

They can be found relatively cheap if you have time to hunt auctions, they're so damn heavy you might need to pour it it's own slab though.