What you have there is known as a ValuVan, the GMC equivalent to a Chevy StepVan. I know StepVan is a generic term since GM has the best ideas and use of names, but you might want to get used to calling what you own what it actually is
I'm into the total tear down and build back up restoration, but it sounds like you are making expensive plans for much possibly unnecessary work and expense before turning a nut and $15k sounds really shy of what I think all that will cost. Why do you think the wheel bearings are bad from sitting? They are a sealed units living inside a wad of grease. Grease seals out moisture better than about anything and sitting doesn't wear things out. That's just the first thing that caught my eye. You seem to have all the emphasis on cosmetics and skipped right over the mechanics, yet worried about bad wheel bearings from sitting 15 years.
It is best to have a thorough plan. But there is a certain element of see as you go involved with a sensible restoration/build. You just don't know what you're getting into till you get there. Surprises can be good or bad. I'm not discouraging you at all, just advising to be flexible with your plan as needed.