Derick, i'd say given some of your other joint + flexibility issues, you may want to avoid leg extensions. They place a rather different strain on your knees and i'm thinking it may not work that well for you long term. I've got knee arthritis, and I do them (and fairly freakin heavy, was doing single leg ones for 110lbs) but as was said you'd be better off investing in something squat related.
something like this perhaps
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And honestly, given the timing, you may want to put off buying it for a couple months. craigslist is basically flooded with exercise equipment from february thru march, as all the new years resolutioners realize they haven't touched the damn thing since buying it for themselves for christmas. So prices are most negotiable then