Thanks for the great advice, EastCoastCB! I'm feeling really good about this bus!
I got the liability insurance today from AIS, who were super helpful and made it so easy. It's $800 for the year ($67/month). I was hoping for cheaper, but she explained to me that the liability is measured by the size and weight of the vehicle, not by the number of passengers.
We're going to pick the bus up tomorrow morning and take it directly to PennDot for registration. I'm expecting them to register it as an "Omnibus" which is a Pennsylvania designation for personal buses that aren't used commercially or for schools. See the types of bus plates here
I'm not sure how much the registration is going to cost. It's either $432 if they count it by passenger capacity (32 passengers), or it's just the regular $37 that they charge for standard registration. See the PA registration fees here
My next worry is about inspection. Its inspection expired in november, but supposedly newly registered vehicles are allowed two weeks time to get inspected. Not sure I'll have time to do the inspection tomorrow, and not sure where one gets a bus inspected, either. I'm leaving for vacation on Thursday, so, if I can't get it inspected tomorrow, it'll sit with an expired inspection sticker until next week. Philadelphia Parking authority will almost certainly give me a ticket.