1. As long as it isn't a basketcase, any bus should be able to go 15,000 miles. A diesel would be preferential. Look for good tires because 15,000 miles is more than the 10-20% tires a lot of buses get swapped on before they are sold can handle. I would recommend a rear engine transit style bus since you're going to be driving a lot. They ride better and will be quieter. Perhaps you can join the Saf-T-Liner Club that keeps getting bigger and bigger.
Engines you'd typically find in those buses are the DT/DTA360, DT466, Cummins 5.9, Cummins 8.3, Cat 3208 NA/T, Cat 3116, Cat 3126, and yes, I've seen a T444E. I think all would be fine choices. The DT466 or 8.3 Cummins would probably be the best given their enormous size and reputation in medium duty trucks.
It might be worth your time to look for a bus with an Allison MT643 transmission. They are stronger, but more importantly should get better mileage and top speed. There isn't anything essentially wrong with the AT545, but you're going to get reduced mileage and top speed.
2. Good Sam's offers tow insurance. My State Farm plan also has it for something like $10 per year. With many places charging a $250 hook up fee and $2.50 per mile, it just plain makes sense to have the insurance.
3. ~$200 per year through State Farm registered as an RV
4. I have minimal plumbing right now, but am slowly adding more. However, showers are easy to find if you look. Truckstops are cheap and easy. Ditto for staying at a small campground if you have some extra time. The electrical hookup will give you a chance to get your batteries properly charged up too.
5. Lumber is expensive. Copper is expensive (wiring), Flooring is expensive. Still, I have nowhere near $5000 in my conversion (yet) and it's very livable. Shop around, decide what you really want, and plan so you can use stuff like carpet rems. I paid $50 for a never-been-used 3 burner stove with oven. My freshwater tank is a 55 gallon plastic drum, etc. Become a member of the FreeCycle group in your area, look on Craigslist, and make people you know aware of what you're doing. You'd be amazed how many people will say, "You know, I have this ..... that might work well in your bus. Want it?"
6. Biodiesel can go right in your tank as is. I don't think you're going to see much benefit. In fact, it will likely be the more expensive option at the pump. Converting to WVO, that is waste veggie oil (sometimes called straight veggie oil), would save you a BUNCH of money, but it is a lot of work. Depending on how long you have between shows, the filtering process could be a pain and take some time you might not have. IT's something I would be thinking about, but it's probably safer just to budget for fuel.
7. A bus will have a GVWR (gross vehicle weight rating) stamped somewhere in it. This is typically in the 30,000 lbs range. It is also going to have a curb weight when converted that you can measure at any truck stop scales. The difference is the payload capacity for your stuff. I would imagine you're going to have a payload capacity in the range of 10,000 lbs. My dad's band hsa doubles of just about all their equipment and it's all 1960's vintage tube style Musicman, Fender Bassman, etc amps with full cabs to match. They stuff is heavy but is still nowhere near 10,000 lbs and I would know...I've had to haul in and out of venue after venue.
8. Wal-Mart is popular for overnight parking, as are truck stops. They will have signs if overnight parking is not allowed. Wal-Mart is convenient for those 8 hour stops because you can resupply, refuel in many cases, be free to come and go when you want, and they're free.
9. With RV plates you should be fine. Check your local laws and see if you need an air brake endorsement for your regular license in your state. Most states exempt RV's over 26,000 lbs from needing a CDL, but find out for sure by talking to both someone at the DMV/Licensing Office and a State Trooper.
10. Take your time and don't get frustrated, but also keep going on it. There are tons of half converted bus conversion for sale on eBay and in the papers. The reasoning is always the same. "I started it, but ran out of time to finish it." As long as you're working on it, the excitement is there.
Welcome to the board and keep us up to date on what's going on. We love seeing projects as they take shape from shopping to maiden voyage!