Looking good man.. I think the 8Hp would do, but it depends on what you want to do with it / Where you want to go. the 18 Oppy is allways good to have ready to swap in. Im not sure if you can just put the 18hp in the same mounting hole as the 8Hp uses, or do you have to modify something to make it fit? i have a 6.5 Hp on my racer.. but I would like to put a 12,5Hp in it, but Im not sure if it will fit.
If you plan to keep the 8hp, you need to experiment with the pulleys, but they say you can not have enough HP on a mudder. Test and see what you are confortable with..