any vehicle thats been sat for this long may need considerable work as unless the engine has been regularly started it may have seized pistons - so 1st thing to check is will the engine turn over by hand -
Also worth draining off the engine and gearbox oil and filling with fresh as these too will have degraded over time.
Also check the condition of any auxiliary drive belts as the rubber will have degraded
and you're going to need to change the battery
Any fuel thats in the tank will be very stale so ideally drain it off fit a new filter and brim the tank with fresh to dilute whats left behind.
If the bus has been sat on its wheels / tyres all this time the tyre will very likely need replacing as the rubber will have taken a set and they won't run true. You may get away with just driving it around for a few days but expect a lot of vibration during this period
brakes will certainly need fresh fluid and don't be surprised if the calipers are seized the system should be bled with fresh fluid at the very least before driving
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