The 7.3 is a very reliable engine. If it has been taken care of you will probably almost never have a problem with it. There are a few simple things to do to check the health of the engine. First thing is to pull the intake off from the turbo and look at the fins. If they are all straight and reasonably sharp that is a good sign. Then take your fingers and grab the end of it and wiggle it up and down and in and out. There should be very little if any play in it. I would test your glow plug relay and glow plugs before winter, the GPR on these tends to go out. Some of us use a better relay as a replacement.
You can also take off the oil fill cap when it is running to check the blow by. There will be a small amount of smoke coming out, this is normal, its just oil vapor. It should however not be excessive. Take the cap, turn it upside down, and set it on the oil fill tube and hold it to keep from vibrating off. If it blows off from the pressure from the tube, your piston rings are worn. If it just sits there and would vibrate off before it blew off, then you have at least a relatively healthy engine.
One problem with 7.3 trucks is the transmission. If it was driven relatively easy, was never over heated, and the fluid changed at the recommended intervals, you most likely wont have a problem with it for a while yet. However the stock 4R100 is not really strong enough to handle a lot of abuse from an engine with this much power. They aren't as bad as a dodge tranny, but they still aren't that great. They can be prolonged for a long time with a good valve body upgrade. I'd recommend getting one from Brians Truck Shop out of arkanas. I've put them in 3 of my buddies trucks, and my own until i got a fully built trans from them. The one that was in my truck is now in one of my buddies trucks. They all love it. This of course is all irrelevant if you have a manual.
Besides that there really isn't much that goes wrong with these if they are maintained well. I run the hell out of mine and have never had any real problems with it. Just keep your fluids changed and be smart with it and it will give you many more years of trouble free life. And if you do have a problem with it, I as well as the other members here will be happy to help you diagnose and fix it.