I dont think it takes allot of power to run a sickle blade mower. I remember when lots of mowing and baling was done with the little 8N Ford tractors. We sold allot of sickle mowers for haying back in the day. But.....sickle bars take some pretty good maintenance and lots of blades need to be changed and sharpened, etc. Not many use such mowers anymore.....to many other ways to skin the cats.......IMO. I think a more modern disk or trum mower may be more appropriate if you are shopping for a three point mower. $$$
One thing about sickle mowers is that they do not play well unless on a decent flat field....IMO. If you hit stumps or tree roots, rocks, fence posts.....your gonna break stuff. No fun replacing rivets in sickle bars and guards wear and fail too.
Mowing and mulching with rotary or flail mowers takes a good amount of power......but you can also get narrow mowers or simply reduce your speeds when the going gets tough. That is where a hydrostatic tractor and things like load match shine.
I think your trying to swim against the current on this issue.....lol.