You have to remember that once the tug/glider combination has accelerated to climb speed, the only force the tug has to overcome is the drag of the glider.
Dan, your analysis doesn't sound right to me. There's the weight of the glider to consider too and the work done by increasing its altitude. The force required is M.g.Tan(theta).
When gliding, this is a negative term which when added to the glider drag equals zero for a still air glide. When climbing the Tan(theta) is positive and adds to the tension in the tow-line. 500 ft/min at 60 knots is about 12:1 gradient which would add another 33.3 KgF to the drag experienced by the tug.
I'd rate the ability to climb efficiently at glider speeds and plenty of surplus power as the main desirable qualities in a tug. I've seen the super-falke towing, and that doesn't do bad on 100hp(?).