The 4.3 engine is a great engine with literally thousands of them on the roads still to-day. It has been around since the 80's if not even before that. Trucks hold their value quite well and the price you are paying is litterally a steal for this unit. It will clean up easy enough but I agree with flushing the rad and do the tranny fluid changeout as well. Use good fuel and it should serve you faithfully for many years to come. The 4.3 is still used in some models by GM as well so parts are easy to obtain and fairly priced as well. 
Well, an update, finally.
We didn't do a swap of vehicles after-all; it turns out that the guy who was interested was only stringing us along trying to keep another deal going with the dealership, but it turned out even better in the end anyway. I sold the Voyager privately for a good price; actually it sort of sold itself, being that I keep my vehicles super clean and well-maintained, and we have officially purchased the truck. I ended up getting $600.00 more for the van than I paid for the truck! So I made money moving up 5 years in age on the vehicle!
It needed, and still needs, cleaning, although I spiffed it up enough to drive it. I didn't wash the engine compartment until we had a few days of above zero temps. I had the rad flushed and the tranny flushed; although the tranny fluid looked very clean, I still had it done. It needed a rear license plate lamp to pass the safety, that's it.
I can't believe how good it drives. It is far superior to my brother-in-law's Ranger, and much better than my mother-inlaw's '92 Chevrolet Silverado. I'm really impressed, so far, with the performance of the 4.3 V6. The mileage on the truck is better than the Voyager, with better get-up-and-go.
The only downside so far is the size of the fuel tank - more than 130 liters, which cost me almost $120.00 for a fill! Although in the truck's defense, I will say that I got almost 700 kilometers of city driving from the first fill-up. I assume that if I do any highway driving at or near posted speed limits that I should be able to get 900 kilometers from a full tank of gas. That's Winnipeg to Swift Current, or Winnipeg to Thunder Bay, on one tank of gas.
I will post pictures soon.