1998 Cadillac Catera timing belt

I am interested in buying a catera but the timing belt has been taken out and needs to be replaced my question is what is the best way to go about this. Does it have to be done by a cadillac service center with original cadillac parts or can it be done @ a regular garage with after market parts. The car is being sold for 800. And besides the timing chain is in ptetty good shape is it worth fixing.