That sucks!
I would seriously talk to your doctor and ask him what he thinks about the front squats. Ask him if it's okay to do dumbbell squats (going down only to parallel) instead.
The dumbbell lunges are woking great for me. Also, if your leg extension machine doesn't go heavy enough, you can use one leg instead of two.
As far as the $200 leg press, if you're thinking about the "old school" one where you lay flat on your back (below), I think that would be horrible for your back. See if you can find an economical 45 degree type.
Chris Cormier is a perfect example of why you shouldn't do the Vertical Leg Press if you have a bad back.
Here's what he said caused his back injury that put him in the hospital.
Chris Cormier (Bodybuilding.com interview):
So, I started training that way. One day I was pushing extra hard, extra deep, on the vertical press for legs. I think I did everything a little too extra!
"By 2 p.m. in the afternoon, my whole body
had spasmed out and locked..."
The same night my legs were sore, and I haven't been able to make my quads sore in over a year and a half so I thought that must have been a h#ll of a workout. The next morning, I wake up and my back is sore. I think that is a little weird and I think, "I'm gonna put some ice on this b!tch!"
The day wears on... ten o'clock, eleven, twelve... Each hour my back is getting stiffer and stiffer. By 2 p.m. in the afternoon, my whole body had spasmed out and locked... I was calling people, screaming at the top of my lungs.