Here is a link for the history of the Perkins Engine:
Doing the swap as you mention even with a "donor" vehicle can and most times run 2-3 times you think it will cost.
Engine/trans, all the electrics, fuel tank to gas and hoses, fuel pump and more.
Like checking out your two speed rear end and making sure the gear ratio will work, drive shaft mods and more.
I know you want the keep the gas engine IH; BUT I would probably rethink that??
Parts availability and cost..
Me I would use a big block Chevy 454 or a Ford 460 for the low end torque.
Reason for these engines is parts for them are everywhere.
My Nickel (inflation) is you are negotiating with the bus owner?
Unless you can get the bus virtually "free" I would pass on it. Because any savings above the purchase price will be gone due to the conversion.
Take a friend with you to play the devils advocate - politely though and point out things you may miss.
When you add up the bus price, donor vehicle price, allot of misc. parts and fabrication price and much more; I would use that money to fine a way better bus and use any left over for the conversion.
Just my opinion; been there and done conversions..