Keep in mind folks, while there's some good info, this isn't another "how many miles do you have on the clock" thread. Yes, these Econolines can outlast a lot of other vehicles on the road, but it depends on engine and how it's maintained, and in some cases, how deep your pockets are.
To the OP, it would help us out if you provided a bit more info on the van you are looking at if you know, if not, these are some things to find out or ask the previous owner. For starters, what engine, 4X4?(adds to things needing attention), maintenance records-fluids, ball joints, plugs and any major service, where it's located-issues related to rust, how its been used-flatlander, mountain driving, off-road, towing, etc.-potential transmission issues. Oh, and pics never hurt! Good luck, hope we can help you out.
'03 Ford E350 7.3L Diesel
(de)SMB'd Custom RB-50
Quigley 4X4 w/Deavers & Agile Offroad's R.I.P. package
CCV High Profile Pop Top