Mark, i guess i could give you a litany of items that should be checked, which would possibly cost more than you are willing to spend... Do you DIY any vehicle work? I do my own wrenching, so i don't know the cost of most of what i am including in my list because do it myself.
Where was the vehicle stored when it was not being driven? If it was in a garage, then it will likely be in a better condition than if not... If you want to post some photos of the below items your not sure about.. Others you can check on your own without paying someone.
On the short list, suspension, rubber components (like Brian said check the coolant hoses, belts ect).. Bushings and ball joints, as well as brake lines and power steering lines, pay special attention to the connection to the power steering reservoir, this is a common failure point. It is possible that the rubber could be rotting due to lack of use.. (you can check the bushings/coolant hoses/belts yourself for dry rot)
I would also pull the valve cover to check for buildup, again due to lack of use/improper use.. in a 5 mile trip the oil in the sump will never get to a normal operating temperature, and can gum up the valve train. Also, you may need a new valve cover gasket as well, if the gasket is leaking, then you mechanic should check the valve train, since he has to take the valve cover off to replace the gasket... he should be looking for sludge/carbon buildup in the head.
Get a compression test done, this will verify your valve rings, as well as your drive train, if you have low compression numbers then something may be going on. this should cost about 100$
Personally, i would replace ALL fluids in the truck.. Everything, transmission, differential, coolant, brake, steering... I would also replace EVERY filter as well. fuel, and air filter should be the ones left as i'm sure you put a new oil filter in when you did the last oil change.
I would also pull the thermostat, and check for build up.. This will cost you a new gasket +labor... Or just put a new thermostat in.. It may be possible that the trucks coolant never got up to normal operating temperature either, depending on how quickly that truck heats up.
Run some injector cleaner though the system... Lucas is a good brand.
To give you a view on my perspective you can look at the below thread if you like.http://www.m42club.com/forums/showthread.php?t=12099
I hope this helps Mark, if i think of anything else i will add it... Richard may have some input as well.