Tom
I've been towed many times with a 180 h.p. 172 and it does fine. We are towing from a 5,000' paved runway (elevation of 200 msl) and I'm flying without water in an LS1-d. The towplane also is used to tow the club's G-103. The Cessna has a STOL kit which, I think, includes drooped wing tips, a modified leading edge, stall strips, and a climb prop. All interior bits and pieces remain in the airplane.

I can't say how it would do pulling a 15m or 18m glider filled with water.

As others have said, the good thing about a 172 is that most any pilot can fly it and that is why the club as a nose-dragger rather than a Pawnee or other conventional-gear tug.

Ray