The 99 is 3/4 size and takes a class 66 bobbin - you can do free-motion quilting on it, but I think the horizontal bobbin would mean that you'd get a lot more skipped stitches than with a vertical bobbin.
The 15 is a full-size machine and the treadle-able versions of the 15-91 are the 15-88 and 15-89. One of those is the model number for a hand-crank - can't remember which one - but converting it to a treadle is easy. You just remove the handcrank and put the machine into the treadle cabinet and put the belt on it. There are earlier models of the 15 (the 15-30 is the only model number I can think of) that were also sold as treadles and handcranks.
As to what machines will fit into your treadle, you need to know the dimensions of the bed of the machine, the diameter of the hinge pins and the measurement between the hinge pins from center to center.
Do you know what's wrong with the machine that's in there now? Some of these machines are unbelieveably easy to restore to working condition. :)