Price and process of rewiring a bus to run from solar power?
Even if you go all electric, you'll have to either rebuild a much less safe and heavy vehicle out of carbon fibre composites, or accept that a ton of your power is going to be coming from the grid, or you'll be limited to stopping places where you can set up a large array of panels to recharge the bus.  Might take a few days or a week with enough panels.  

If you're keeping your gas engine and just want to charge the battery with solar in addition, that's not such a big deal.  Wire panels to charge controller, increase bank size if necessary.  

So for the question you appear to be asking, 

What would have to be done?

Rebuild omnibus in carbon fibre composite and aluminum, improving aerodynamics and maximising sun exposed area.  Eliminate all non essentials.  Plan where the bus will be travelling.  Purchase, mount and wire panels suited specifically to those lighting conditions.  Design lithium-ion battery array and charger that is somehow safe inside carbon fibre and aluminum deathtrap.  Cautiously connect battery.

Specifically, what would it entail to completely change how a bus runs?

To some degree, vehicles are designed around their engines and fuel.  If you're taking out the engine, see above, that's going to be a complete redesign.

Rough estimate pricing?

Lots, man.  Like, so much, like, some more a lot.  It would cost a substantial amount of money to make a decent estimate of the cost.

As far as the more reasonable question of adding solar, keeping the engine and increasing the battery bank, meh, couple grand, maybe as much as 50, depends how many batteries and panels you can safely fit and what type of panel, and fit and finish of course.

As far as sources, I recommend you shop around to get an idea of what the parts can do.  Search for the best panel, the cheapest panel, the best deal, and dig into the datasheets, learn about light conditions, figure out how many of each panel type you can fit on your microbus(film types might let you keep a sleekish look while fitting more panels if you have extensive autobody skills), and how much power you'll be able to generate at peak(brightest light, best angle) and over a day.  Then you have a rough idea of how much power you might have and you can figure out what you can use it for.  Once you have an idea of what you can and want to use that amount of power for, start figuring out how much power you'll use and when, because the longer you store it, the larger your battery bank must be.  If you want to still have your fridge or lights working after 4 days without sun, you need a much larger battery bank than you would need if the battery only has to last overnight.  

You'll end up with a tradeoff with space used by the batteries and come to some compromise, and aside from the fact that the engine lets you move the thing around, at some point you'll be glad for the backup generator.  Or get an actual backup generator designed for the purpose and leave the alternator to charge just it's own battery.