Cost.... Buses are relatively cheap. I picked this one up off ebay. $3K. It has a big 5.9 turbo deisel (Cummings) with an Allison auto trans. Both pretty much bulletproof. It has 96,000 on it, a drop in the bucket hopefully. I drove it to Maine. I like Bluebirds, rear engine (much quieter on the road), MUST be deisel. Keep an eye out for rotted floorboards. It's not a huge deal, but it is a pain. Living in Maine, I rip everything out to the metal and start from scratch with a vapor barrier, thicjk insulation board, then the subflooring.
There are some hurdles in terms of weight and CDL needs. The bus is 30,000lbs. with air brakes. In Maine, I needed to throw a mattress and a portable toilet in the back to get it reassigned as a motorhome. Once I got new plates and registered as a motorhome, no CDL is needed. I did recieve a transportation temp. permit to get it home without a CDL.
Insurance is through GMC, cheap... about $400/year.
I plan to drop 6-7K into it. I want to keep it to around 10K total. It is doable if you get creative, hell you can do it for alot less if you get really creative. My biggest overlying concern is weight. I take that into account with each and every step.
Rain collection.... I have thought about using a temporary tarp system to collect the water. I will have a 50gal storage tank for potable h20, 50gal. another tank for unfiltered h2o, another 50gal for grey water. I saw a guy who stored rain water in 2 big sealed P8" VC tubes that were mounted on each side at the top of the bus. He then used gravity feed to bring the water down inside the bus. Not sure if I'll do that, I may just feed the rainwater into the unfiltered water storage tank. Something else to think about.