Well I just bought a high mileage Bonnie 156K, and so far it needed a new oil sending unit and a t-stat for the cooling system. Also the LIM gasket is leaking coolant, so just check for any leaks under the car, especially oil and coolant. It can be fixed as I am finding out but although going to be quite expensive if you take it to a mechanic which I will in my case for the LIM problem. Other then that make sure the engine is good, if they will let you take your car to a mechanic before you buy it most places will check out your car for free. Good luck, I am extremely happy with my Bonnie with 156K...I plan on making it last with some tlc.
2001 Pontiac Bonneville SSEi with 160K (Retired)