Check out this link, you should be able to find out some useful information.
www.budoseek.net/
I have not heard much good about the SCARS system, but since I have never seen or tried it, I can't say.
I can tell you that if you have bad knees, Aikido may not be a good idea. We train in a kneeling position, as well as standing.
You may be able to adapt, but make sure that the instructor knows your knees are bad, and go from there.