Usually stress on the knees from cardio exercise is caused by jumping and pounding from high impact aerobics or running. I have knee problems and I never experienced a problem from an elliptical machine EXCEPT for one instance. Right after an injury, my knee was causing me pain if i straightened it completely. I had to be careful on the elliptical machine not to let my leg go completely straight or else it hurt. It wasn't a problem with the machine itself but just something related to my injury.
As far as price goes, this is where I lament. Good elliptical machines cost $$, therefore I visit them at my local gym. If you want one, prepare to spend $1k or more. I don't know where it is but there used to be a website that sold used gym ellipticals for $2k and that is probably the direction I'll go towards when I do decide to buy one.
You can't out-exercise poor eating habits.