You should be able to run a 15' donut with a 1/6 hp pump--I did for 3 years. You must be careful that it seals and that you keep the filters either clean or new.

If you must go with a bigger pump, look for a 1/2 hp UPrated pump, not full-rated. You want the smallest least powerful pump so it doesn't rip the fitting out of the pool wall.