High mileage doesnt mean it hasnt been looked after, unmodified are a bonus. as long as turbo etc are ok.
You will also find that the problem parts have already been replaced, fsh is what you want to look for and bills for parts that have been replaced.
Get someone to follow you in your car..
Ask if you can give it some welly through 3rd say right to the redline and ask a mate to watch for any blue or white smoke during acceleration and when you change gear (thats if the car behind can still see you
)
Some black smoke during this exercise is normal in small quantities and in any case is just un burnt fuel.
Check the gears are all smooth and there are no noises in neutral, boxes are not cheap to repair and are more likely to be worn on a powerfull car.
Going back to the engine, let it idle so it gets hot enough so that the fan comes on and goes off.
Ask the owner to not start the car before you get there thats an old trick to disguise smoke on a cold start up...
Check the tire wear make sure its pretty even if its not it might have been in a prang. Look at the camber on the rear wheels, if there seems to be more that useual the radious arm bearings may have gone, also listen for knocks when driiving for this..