IMHO - it is a black box right now... could just need a timing belt, could need a rebuild.
Frankly, amount of wear on the tires would be a minor concern right now - do you have budget to replace the engine if it's that bad?
This is a gamble but don't play unless you can afford to be on the losing end.
Would this be your 2nd or 3rd vehicle? Can you manage with it in the shop for several weeks?
Figure the cost of a full engine rebuild + the $2000 purchase price + any other items that needed attention. If the vehicle was that pristine, I'm not sure I would let it go if it "just needed a belt" - even considering the labor involved.
2006 Granite S