50 lb Thrust Motor on a Intex Excursion?

I am going to buy an Intex Excursion inflatable boat. On average I'll have about 400 lbs on the boat although at times I may have up to 700 lbs onboard. What size motor would be sufficient to travel at a decent, non-crawling speed? It's solelyI was thinking of getting a 50 lb thrust Minn-Kota. There's 30lb and 40lb also for sale, the 30lb is significantly less money. Thanks!