One thing to consider here, with a family that size and full timing in it, you will need a lot of storage space, something that is very much lacking in school buses. I have seen a few out there with a storage compartment underneath it, but even those are not all that spacious.
You'll need plenty of seating for travel, as well as room for the entire family when parked, think about where everyone will need to be or what they will be doing on a rainy day. Sleeping quarters (and I'm assuming we're talking about lots of bunk beds here) also takes up space.
Front engine or rear engine? Different folk's have different idea's on that. Rear engine buses have certain advantages... first is the noise issue, with the engine all the way in the back, you don't have thing roaring right beside you as you're tooling down the hiway. Rear engine buses generally speaking have easier access to the motor compartment too. When it comes time to service/fix the motor the grease and oders will be out back, not in your living room.
Gas or Diesel? Again, different choices for different people. You'll get far more service life out of a Diesel than you will a Gasser, generally more power too. However, right now gasoline is cheaper than Diesel, but it hasn't always been that way either.
Furniture? I have used a mix of RV specific furniture and regular house hold furniture. You need to use what fits.
Space? I'd definately look into building slide-outs, they will increase the interior living space but quite a bit. Just having enough floor space so that 2 people can walk by in different dirctions at the same time will be important, especially with a crowd like that over a long period of time.
For what you are doing, I don't think I'd even consider a School bus unless it already had the raised roof and the lower compartement storage. I think you'd be better off with a hiway bus like an MCI or something like that. They are rear engine, have 3 or 4 HUGE storage bays underneath, taller interior ceilings, hiway gearing already in place too. Ebay seems to have plenty of them though prices vary wildly... some already converted too!
Nice MCI Listed here