I have recently had a few of these come and go through my hands. I would say, don't be afraid of one that "hasn't been tested." Many folks don't realize that the battery is in the film pack, so they think the camera is dead if it doesn't spring to life somehow when the buttons are pushed. Do be careful if the particular example you're looking at is beat up. They don't take very well to being manhandled if they're open; the closed camera is a bit more tough, like a turtle in its shell. Also, leather can always be glued back on if it's coming loose but if you see mildew or mold consider passing; that would obviously speak to poor storage conditions that may have affected the innards. If you're depending on photos to make your assessment, look for any parts being bent out of shape or somehow out of alignment. Other than that, it's a crapshoot! I will say that every one of the SX-70s I have had recently were perfectly fine once they had a fresh pack of film. The Sonar model (my current SX-70) is unwieldy and not as streamlined, but my copy works amazingly well. If you can pick up flashbars along with the camera, grab them - they aren't made anymore either and other aftermarket solutions for the SX-70 flash are hit-or-miss.
Also, I'll say this about Impossible Project film - some folks like it for its 'dreamy' qualities - you likely won't be getting razor-sharp images from it, and you will likely burn a few expensive shots before getting the hang of it. Even once you 'get it,' it does get expensive to run. I use mine as an occasional treat but if I want to use a polaroid at a party or something, out comes an old Polaroid Colorpack II with some new Fuji FP-100C film. The pictures are better and the film is much less expensive.
All that said - they are wicked cool and quite a conversation starter. Good luck and if you find one, enjoy it!