1997 Pontiac Montana Should I buy this Vehicle

Be prepared to do some repairs, maybe even major rebuild or replacement of transmission or motor with this kind of miles.
Best to tell seller that you want to have it checked out by your mechanic and ask if you can take it for a full day or weekend to test drive and have it checked out.
Have someone you know and trust to inspect it, even if you have to pay $50-$100.
I would find something with less than 150,000 miles and spend up to $3,000, dont get in a big hurry to buy. Take the time to check out cars for sale "by owner".
The main things to look for are: Quiet engine with no strange smells or noises, no oil or fluid leaks, that dont smoke when excellerating, no obvious signs of hard use such as towing hitch, service record of any major repairs will be a plus on high milage cars.
Negotiate, never give asking price without first giving an offer of 65%-75% of asking price.

Being a current owner of a 1999(purshased new) I have had to replace the following:

intake gausgate and water pump (common problem) = 1200.
A/c cluch = 400
gas lines (rusted so normal wear/tear) = 200
rear brakes because of leaking cylinders (common problem) = I forget price
paint off of the hood (since the hood is aluminum GM did not primer it properly but they wont fix it) = iforget price

so would I buy another, probably not. If you were to buy, make sure that the intake gausgates have already been done.

Steve