I think that 80lb thrust should be enough, I have a 65lb thrust on my 18'seapro, the speeds go from 1 to 10 and I rarely have to set it over 6. But like some of the others said, I think the shaft length is the most important, the basic way to find the right shaft length is to measure from the top of your bow (Where the TM will be) to the waterline, then add 20". my boat actually came to about 60", but I found a good deal at boaters world on a 54" shaft so I gambled and bought it, I rarely ever have a problem with the prop coming out of the water, sometimes when it is rough out and I set the speed to 10 it will. I bent the bowrails up on mine for clearance, but you need a gripglide TM for that, I,ve also seen people cut the front of the bowrails off.
also, when I was shopping around BPS had some good deals on reconditioned TMs, you may want to look into that.