What kinda exercises do you mainly do? Running? Walking?
I'd suggest an Elliptical Trainer as the overall workout is fair superior than the treadmill and not as hard on the joints....but you can elect to just take it slow and just have a cruisey walk.
I'd also suggest one with some programs or programming facitlity...mine has an *undulating hills* setting and the arse burning *Mountain Climb*....that bugger makes my Gluteus Maximus scream for days.
Second hand is the way to go, I dont know what they run for preloved over there, but over here its easier to buy them 2nd hand than rent them for a couple of months to make sure the enthusiasm doesnt wear off too quickly.
I have a shed full of exercise crap that I have bought and then lost interest in, the Elliptical Cross Trainer, my medicine ball (rather than weights) and Exercise ball as the only ones I have stuck with...besides a skipping rope