Also see: Diesel Vs Gas
Also see: Transmissions
Never under any circumstances go underneath a bus with air suspension without blocking the bus first. A failure of the suspension will cause the bus to fall to the ground and it will crush anything and anyone underneath.
Air suspension will provide a softer and often adjustable ride height and quality at the cost of being a long term maintenance item. Generally the air bags will need to be replaced approximately every 20 years. Air bags will cost around $150 each. With minor modification air suspension can also be used to level the vehicle when parking by raising an lowering each section. Leaks in the suspension can cause the bus to slowly "fall" to the ground when parked. This can be avoided by maintaining a leak free system or by using a small auxiliary compressor to keep the air system full. An air bag with a severe leak or blowout can cause that section of the suspension to immediately fall to the bump stops. In this event the bus should be immediately taken off the road and the air bag replaced.
Conventional Steel Springs
Conventional steel leaf and coil springs will provide a harsher ride in an empty vehicle and will soften as more weight is added. They generally require no maintenance for about 50 years unless severe rust has started separating the leafs or rusting the U bolts. If there is severe rust the leafs will slowly increase the load on the U bolt that has likely also been rusting. The end result can be a snapped U bolt which will detach the axle from the vehicle. This separation can be mitigated temporarily by using a sledge hammer on the leaf pack to break up the rust between them and let them settle back together. This is only a temporary solution though and the U bolts should be replaced to avoid a failure.
PSA: No matter the mechanical condition of the bus you will likely need to purchase new tires. You will want to incorporate that cost into your initial budget. You don't want to start your bus journey by blowing out two tires, having an accident and tearing up the side of your new to you bus.
Good tires are expensive. Do not be surprised if you spend 1500.00 or more (for a shorty) or multiple thousands for a real bus.
Lifts (Handicap and more)
Having a lift in your bus may be a must have if you or someone you plan to bring along for the ride is handicapped. If not, it isn't necessarily a bad thing to have. Depending on the weight limit of the particular lift they can be extremely useful during the build process to move large items like appliances into the bus. A handicap lift also means you have a larger side door that may be required depending on what you want to bring inside. You can often sell the lift for a decent amount of money if you don't want it which will recuperate some money and give you the large side door.