congratulations on the twins - we have a set, they are a BLAST.
We used a 'standard' minvan until we grew out of it. Now own a Chevy Express 12 person van, 3yrs old with 50k, for 18k iirc.
We considered used bus (school, airport transfer, etc) but
- tend to be high miles, rough life
- not pleasant, esp on long trips
- huge to park, limitations due to height, etc.
I can sure see the advantages of the extra interior size. A co-worker (also with many kiddos) recently 'got a deal' on a new Ford Transit 15-pass hi-roof. It's a cave in there.
Parts and service should be no issue if DH is handy, and you purchase something reasonably common
Interested to see what you do. Keep us informed! :)