When looking for your bus, think long and hard about what type of roads you will be traveling. Will they all be highway? Will you ever hit an interstate speed? Will you ever travel dirt roads? Keep these things in mind and be flexible - willing to compromise - on your final "dream machine".
A 40 foot bus sounds awesome with loads of room for living, but you do NOT want to drive one through a small town with narrow streets if you are unfamiliar with a vehicle that long
! And trying to make a sharp turn with a tree, boulder (or another vehicle or lamp post, for that matter) in the way will be a major bummer.
If you hope to stay in campgrounds, consider the size limit some impose because they simply aren't built to accommodate a long vehicle.
Once you're sure what you feel comfortable driving, keep you eyes open on both Craigslist, ebay, and your local trade newspaper. I found my first bus on Craigslist. While I had to sell it, I'm looking for the next one now. In the mean time, plan, dream, sketch, and change your mind a dozen times. Look at the photo gallery here for inspiration. Don't be afraid to think outside the box. Not sure if something will work for you? Build a mock up out of cardboard or scrap wood to get a 'feel' for things. Once you're fairly sure what you would like to do, you can start to gather components now, thus spreading the cost over time while looking for the bus.
Most important of all, remember you'll spend more than you expect. More money, and more time. Life happens. Just don't let it completely stop you.