1990 model International with an 8 cylinder diesel. ....
I would suspect that you are looking at the old IDI 7.3.
With the exception of chronic glow plug issues they were very reliable engines.
Unfortunately they are also quite underpowered in a bus applicationl. If you plan on much highway driving or mountain driving, I would keep looking.
The replacement for that engine in 1994 (?) was the T444e. A much better choice but not my first choice in a full size bus.
Brokedown successfuly traveled the country in a T444e powered, mid-sized, AT545 equipped Vista. It can be done. Personally, I don't have the patience...... Give me a gob of torque, 235 horsepower and an overdrive transmission..
If you watch the auction sites you will see well equipped buses for $1700-$4000 if you have a little patience. I found my 2002 Bluebird with 8.3& overdrive transmission for $3800 including auction fees.
Good luck with your quest.