Tremmor,
I am a new AATV owner and I can assure you they are a blast. But, you need to fully evaluate what you are getting into, because that $700 for a set of tires could be the tip of the iceberg. You really need to see inside that machine because it looks like for the 10-15 years its been sitting it has been taking on water (unless its under a cover). If the tub is holding water you could have significant repairs/replacement with chains, sprockets, idlers, bearings, frame, and so on. I"m not trying to discourage you........just trying to give you a dose of reality.
There's nothing but help on this site, and it's kind of like Home Depot.....you can do it, we can help!