The problem with the van is that it is already warning you that the fuel pump is going to go south on you at any moment. There is going to come a day, not far off, when you are going to bang on the fuel tank and it still won't run. Fix the problem in the tank before you do anything else. If you don't get fuel to the motor, it won't matter what condition the spark plugs and other things are in. And the longer you try to drive it with the dirt etc in the system, the bigger the chance you are going to damage the rest of the fuel system, injectors, etc. Once you have the fuel pump replaced and the tank cleaned, then you can worry about doing the tune up and improving the fuel economy. But, no matter what you do to a 3/4 ton van with a V8 motor, it is never going to be economical to drive. If you want to do this work yourself at home, I would be happy to guide you through the job. The hardest part is getting the fuel tank out so you can work on it. The less fuel in it at the time, the better. It is also a real good idea to have a helper for lifting the tank in and out of the truck.
As for the Voyager, without having a chance to see the smoke, I would tend to worry. White smoke is normally a sign that the motor is burning radiator coolant, because of a leaking head gasket or a cracked head or something of that nature. As to the oil leaks, it depends on what is leaking and what repairs need to be made. If it is only valve cover gaskets or an oil pressure sending unit, etc. they are pretty cheap and easy. But if it is a rear main seal or an intake manifold etc; then your looking at a lot of work and pretty much a good deal of money. Really depends on how much of it you can do yourself and how much you have to have a good shop do for you.
Drop me a note and let me know what you decide to do, regarding either of these vehicles and I will be happy to help you with what ever you decide. But, your really pushing your luck and defying the odds by driving the Chevy when you know it is developing a problem that may get worse with each mile it goes. Thanks