The only advise I can give you is to do some further investigating. Try to get your hands on maint. records if they exist. I have a 2001 250 and work for a company that has about 12 powerstrokes; depending on who drives it and how often and well it's serviced you could get a good or bad deal. You really should drive it to make the best decision: Most of the PSD's i've driven have over 100k miles, some run just as well as mine which has 31k, and others are completely starved for power. If this is an online buy and a test drive is out, here's some questions that will clue you in to the performance of the truck...
*Has the truck been used for regular towing?
i.e. does the truck drive to and from the office, or does it pull a heavy trailer every day? If it pulls, at 240,000 the truck has alot of work under it's belt.
*Has the transmission recieved service other than basic fluid change?
-If the tranny has been rebuilt or replaced multiple times, it's possible that the owner drove it a little hard.
Neither of these questions are supplements to a test drive. It is true what you have heard, PSD's last for a very long time; I've driven one with 370k miles and it still chuggs along with no issues. AS long as the truck has been serviced well and looks to be in good shape, you'll make out fine.
Happy shopping! send pictures...