I'm thinking about buying a 1991 buick regal it has a ticking noise coming from the rear of engine. older gentleman owns the car that's in great shape, 42,000 miles. not sure if it's an expensive repair and worth putting money into?

Difficult to say for sure without having the vehicle present... However that is likely caused by a valve lifter and very well may just need an oil & filter change with some Marvel Mystery Oil added, then blow the soot out of the engine by giving it a good old fashion aggressive test drive!.. Older folks 'usually' tent to just put around and don't exercise the engine enough... Driver will most likely see that crap comming out of the tail pipe upon hard acceleration .. I have some older customers at the shop and we usually give the vehicle a pretty good work out, if needed, but we're in the country with some good roads to do so without much danger.. Don't get a ticket by trying my suggestion! .. You didn't post engine size but those V/6 engines have proven themselves to be very reliable... (3.1L ... 3.8L) .. It should be a bunch more life left in that vehicle! .. They good cars!