The HOB P-51 is a great kit. There is a fair amount of sanding to be done and the engine compartment is rather cramped, but other than that, it's a pretty easy build. The plane flies great. I don't think you'll have any trouble with it.
I had a Fox .19 in mine. If I were to build another one, I'd mount the engine upright or at 90 deg instead of inverted.
I wouldn't use anything larger than an OS 25LA or a Thunder Tiger GP 25. Both are inexpensive and will be a good match for the plane.