I am towing with a Sr. Telemaster. Mine is electrified, weighs 15lbs. I have towed 18lbs gliders with that one. You may also use gas power with the Sr. TM, of course. Some glider buddies of mine have built the AMR Trainer 50 (from a kit), we just towed gliders up to 41lbs. on electric power - see video below:
http://www.youtube.com/watch?v=XqIirv-w9Qs
http://www.youtube.com/watch?v=42hHJbM2CCA
http://www.youtube.com/watch?v=-8cyt4rmneg