Ah. Copied that. Welcome to the OBS part of the forum.
I had a '99 SD, and sold it. I really liked it, but circumstances dictated...
I just sold my '95 and I still have a '96, so I like 'em, but I'm biased. I also believe the 7.3 is a better motor all around than the later motors. I've heard stories and rumors of stories of guys bringing their pickups with 7.3's in them to dealerships with over a million miles. I've seen more than a few guys with over 300k on this board.
Look for the usual stuff; leaks, smoke, easy starting. Maintenance records would be a plus. Rolsmojave3 says to always tell the seller that he won't buy it if I can't touch the exhaust manifolds when I get there. That engine better be cold. That way he gets to see how it will start cold. Good idea, there.
I understand some of the older programmers are harder on the transmissions than the newer chips, so that might be something to avoid.
Superduty4x4 made a good post on this:
Major items to check-
Pull the intake tubing off with the motor cold and try to wiggle the nut you can see inside the turbo- a little side to side play is normal (as long as the wheel doesn't hit the housing), ANY in and out play is bad. The edges of the blades on the turbo wheel should be relatively sharp and straight- no nicks, gouges, excessively rounded edges, etc.
Check the intake tubing (especially the red boot at the turbo if it still has one) for cracks.
Get a good flaslight (not one of those small LED pieces of crap) and look down in the valley of the engine- there should be no puddles of any liquid in there.
Depending on outside temps where you are, check for white smoke immediately after you start it. If its cold out and the truck has a hard time starting and smokes white then the glow plugs are probably bad (remember, it's a diesel and if its in the 50's or lower give the glow plugs a few seconds to heat up before trying to start it).
Check the coolant for SCA levels if at all possible
Those are the major items. Of course don't forget the regular stuff like brakes, ball joints, tie rod ends, tire wear, etc. If you can, take someone familiar with Powerstrokes with you. Someone with access to something like AutoEnginuity would be a big plus.
Just stumbled across this post. It covers quite a bit:
What to look for when buying a used 94-98 PSD
Another one, specific to '99 - '03's, but it's still a 7.3:
What to look for when buying a USED 99-03 PSD
Keep us updated in your search.