Leg press machine for rehab? Good design?

Approximately 6 years ago I ruptured the anterior cruciate ligament in
my knee. I have managed to stabilize using cycling and weight training.
However, I fell about 2 foot onto it and it gave out causing pain, etc
etc. It feels pretty bad (still waiting for a diagnosis) and I think it
might be months before I can squat my own weight again. I was thus
thinking of building a hack squat/press machine, so that I could work
up from weights less than my body weight. I was going to build as they
seem soo expensive.

Which exercise would provide less stress for my knee? I have seen talk
of the hack squat producing more shear forces (think I should avoid
that!), is that the case?

Some machines like the

Show the backrest slightly curved at the lumber region, while others:
http://www.gymworld.co.uk/prodpage.asp?ProdID=243 would appear to have
a flat back. Which is correct?

Do the same rules apply as in the squat, ie knees don't extend beyond
the toes? Is the ideal position/range of movement for the knee of
90degrees to 179? With the ankle calf angle staying as close to
90degrees as possible?

Many thanks for any help given