I'll have to make up a list...but maybe I will start on it...
Check all the lights. The marker lights in front especially because they are a pain to change.
Check clutch and brake pedal play. If your getting excessive grinding or are having to really push the clutch down to get it started, check under the dash where the clutch lever meets the master cylinder on the firewall, there is a little plastic bushing that mates them together and it wears out....an easy fix/modification if there is a lot of play between the lever and the rod. About $50 to modify it.
Take the intake boot off from the turbo and look at the turbo wheel. It should be clean and mark-free. Chips, cracks or chunks missing from the wheel, or dull edges means dirt has gotten into the turbo and really shortened the lifespan of the truck. If the truck has been fitted with a K&N filter, take a real good look at that wheel because the K&N filters let in too much dust. K&N good for gas, bad for diesel.
Roll around every which way and look at the body. If the truck has running boards on it, that will have saved the edges of the cab a fair bit.
Look at your crossmembers under your bed, see how many are rusted out, if any.
The 5 speed ZF trannys in those trucks are pretty much bullet proof...but take it for a drive and try shifting....also try downshifting from OD to 4th and then to 3rd and make sure it shifts smooth, just keep your rpm's under 2000 while doing it.
Get a truck with manual 4x4 locking hubs. It's a pain to get out and switch them by hand, but you'll eventually want to replace the automatic hubs with manuals anyways.
Find out how old the batteries are, look at you battery cables, especially the positive cable clamp on the passenger side as it has a tendancy to crack. New positive battery cable is about $140.
Find out what the owners interval is for changing oil, and what type. 15W-40 is what is for those trucks and should have been changed at an interval of no more than 5,000 miles. Also make sure he changed the filter too. See what kind of oil filter is on there as well. FRAM are bad. Purolator, Wix, Baldwin and Motorcraft are you best choices.
Run the truck, open the hood, check the oil level, and take the oil fill cap off while the truck is running. This is to see how much exhaust is coming through the oil pan. Rule of thumb is that if you flip the cap upside down and place it on the hole and the exhaust coming out blows the cap off the hole, the motor is going to have to be rebuilt.
Ask what kind of anti-freeze he is running. The 7.3 has thin cylinder walls and can be prone to cavitation if the anti-freeze does not contain SCA additives (would say on the label). Majority of the ELC (Extended Life Coolants - Red in Color) are fine for these trucks.
Ask if the injectors have been rebuilt or replaced. Average life in injectors is 250,000 miles before being rebuilt. New injectors are worth $1200 and up.
Take a look under the turbo, dip your finger so you can touch the valley of the motor and see if it is wet and smells like diesel. If so, you probably have a leek in your lift/fuel pump and while you can do the work yourself to replace it, it is a time consumer and back breaker. New pump is worth about $200, but the labour in a shop would be easily triple that.
Truck should start within 5 seconds of cranking after the glow plugs ("Wait to start" light) has went off. If any more than this, the starter might need to be replaced ($140).
Make sure the dash lights and everything inside works.
Ask if a front end alignment has ever been done. Check the front end and see if the ball joints and tie rods have been greased. Again, proper maintenance avoids pains like ball joint replacement.
Ask if the shocks have ever been changed. Stock shocks are finished on any vehicle at about 25,000 miles. If you have to change them, it would cost $400 for a good set of Bilsteins.
This should give you a good start on things. Note the prices and judge accordingly as to how much you can wittle the seller down on price. People who take care of their Ford trucks deserve a good price and those that do not, deserve it to be purchased as cheaply as possible to help it get a new lease on life.
You can go to http://www.ford-trucks.com/forums/index.php and sign up for free for the forums and get a lot more tips. I am on there and there are a lot of helpful guys and ex-ford mechanics on there that can give you the info you need.
If so, 1995-1997 Ford 3/4 or 1 ton 7.3 Diesel 4x4. Lots of info on the web for them and I can give you a lot of advice if you start looking at one.