I've redone the upholstery on two 107s. On both I swapped the seat base between driver and passenger side to balance out the leaning. I think people getting in and out over the course of 20-30 years has made the springs weak on the outside. Swapping the seats took care of it.

I've heard of broken springs, but I've never run into one. If I had, I'd take it to an auto upholstery place and see about getting a replacement spring. They're just standard S springs, and they shouldn't be too difficult to replace.

Getting the upholstery off is simple, but to get it back on after the seat is taken care of you'll need a pair of hogring pliers and hogrings. If you can't find any at an upholstery shop, look at a fencing supply company. They use hogrings with slightly thicker wire for fencing. A 3 lb. bag of hogrings and a pair of hogring pliers cost me a whopping $20, and I've still got enough to do three or four more cars. Buy the thinnest wire you can get, with sharp points to puncture the material.

Details and tips are included here:
http://www.benzworld.org/forums/r-c107-sl-slc-class/1315742-new-upholstery.html?highlight=new+upholstery