4) If you can ask the question then you already know the answer!
3) Pricing is usually based on age, condition, and spe'c's. Type 'D' buses (service door in front of the front axle) generally cost $10K-$20K more when new than a bus with the same spe'c's and number of seats as a Type 'C' bus (front engine under a hood outside with the service door behind the front axle). As a consequence they tend to cost more used. The big advantage of a Type 'D' bus is the entire length of the bus is inside rather than having 8'-10' of the bus sticking outside out front.
2) Most school buses that come out of service will have legal tread on the tires. In some cases the school will go through the fleet and find the six tires that have the least amount of legal tread and put them on a bus that is being sent down the road. If you find a bus with a good set of rubber it is a bonus. As far as good or bad, the only criteria that is important to you is if they have legal tread depth and determine how old the tires are currently. Our church purchased a bus for $3,500.00 and ended up spending $3,600.00 on new tires and wheels--when we purchased the bus it had no two tires alike and the drives were all tube type wheels.
1) It takes several hours to remove all of the seats. At normal truck shop rates $700.00 wouldn't even be one whole shift for one person. I know from experience it usually takes two people a lot more than four hours to remove a seat of seats. As far as using them for seats in the pit, unless you happen to purchase a bus with track mounted seating you are going to have to build legs for one end of the seats--the wall side of the seat is usually bolted to the chair rail in the wall and do not have a leg on that end. 2002 vintage Type 'C' school buses with an asking price of $4,500.00 is not a bad place to start.