Hi mate, The main thing being, what are you going to do with your tug???? Towing etc will need the extra power, if its just stooging around then possibly lower power, i follow Stavro's formula with extra power you just throttle back.
The spec from the ebay shop shows 450 watts @ 3000rpm, not far short from propshop 500 @ 3000.
Unless you have a scrapyard nearby a heater blower motor or radiator fan motor would be superb for power, torque and watts. they are fairly big but tugs have big hulls. If you can get a cheap blower motor ie less that the ebay motor, go that way,
Hope this helps
Mark