Wow, 3 years on here before your 1st post!
Anyhoo, welcome to the site.
There are so many things to consider before purchasing a bus. Unfortunately nobody can say buy this bus and you'll be all set.
It matters what type of usage you'll be putting it through. Also the climate you're in or plan to use it in, etc.
The best thing you can do is go through the numerous threads to figure out what type of bus will work for you.
The basic advice I would give you is try to get one from outside the nation's rust belt. You don't want to have to fix rusted out flooring and/or structural chassis floor support before starting a conversion.
If you want a diesel powered bus, pre 2004 is what you really should consider for reasons detailed in many threads on the site.