I have pulled 250-300lbs of people on a tube with 3 adults and 2 kids on my boat ok. I had to make sure the weight was balanced and I was going about 35+ with all that. I do have Skat swirls on my boat and thats it.
Neither the Challenger 1800 or Sportster 1800 is that great for a tower. Our boats do have a ski pole on them so pulling a wake boarder shouldn't be any harder. 717 have very good torgue just not real good on the top end.
I have seen people mount the towers on the Challenger 1800, you have to renforce the areas where the tower mounts go so you don't rip the boat apart. No one makes a direct fit tower and if you want a cover for the boat is has to be custom made.
Most people just add towers to add better stero stuff.