I've read (on this forum and on other sites too) that the cooling system of the 540 is notorious for needing a new radiator and water pump at high miles. Have they been replaced on the one you're looking at? Mine needed a new rad at about 132,000 and a new water pump a few thousand miles later (so I twice had the "joy" of being stuck at the side of the road). As Dirty Harry said, do you feel lucky?
Also, make sure that the 5HP30 gearbox doesn't whine when it's cold - I would do a test drive immediately after statring the engine (the whine is in second gear, at about 20-30 mph), then come back and check the car over, then make sure it starts easily when warm. Whining can be low auto fluid or the wrong (i.e. non OEM) filter. The auto fluid is expensive and it's a pig to replace the filter and put in the fluid (you have to fill the autobox from underneath the car!).