A 97 Geo Tracker doesn't have a carb, it's going to be fuel injected (all of the Tracker's were), and the problem is going to be the valve seals, not the guides.
My first question is to you is - what is an "upper rebuild"? If it's a rebuild of the upper part of the engine, then the valve seals would/should have been replaced as a part of the rebuild.
My second & third questions have to do with how honest is this guy being with you, and, what was the quality of this "rebuild" - the vehicle would have to be priced really attractively for me to consider it, and I have the skills to do such a repair myself.
Constant oil burning, can "poison" the catalytic converter and kill the O2 sensor, fairly expensive parts, and if you forget to check & top-up the oil, you can get yourself into big dollar repairs very easily.