Mini Bird may have been Blue Bird's name for these, however, the bus pictured is not quite what I would call a 'mini bird', as it is on a P30 chassis. The P30 is the step van chassis, widely popular with Frito-Lay / Wise vendors, and many have been converted into food trucks. What I would consider a 'Mini Bird' is usually called the Minotaur, which were built on Ford E-vans and GM G-vans.
As for the engine, the 350 is commonly also referred to as the 5.7L, it is in the LT-block engine family of 1955-2000, block and internals remained the same for much of its production run. Heads were revised when throttle body injection was added around '87, but the block's overall architecture dates back to the 50s / 60s, depending on how you look at it. These smaller P-chassis buses could be had with 350 / 454 gas power, or the 6.2 diesel in some cases. The 6.2 was enlarged to a 6.5L and turbocharged around 1992, so if this is a diesel, it should be the 6.5 turbo. Better than the 6.2, but not the best engine out there. The 6.2 is kind of an albatross, but but not the worst engine ever. The 350 is solid and will give years of service if maintained.
A 1998 could be a transitional year, as that was the year investors formed the WorkHorse group and took over the production of the P-series chassis privately. What this means is that for at least a part of the 1998 model year, the chassis was supplied by WorkHorse, not GM directly. I doubt they changed much of anything, but just be aware that this chassis may not have been factory GM.
However, two things you should look for with this particular one.
Being a 1998, it is likely to have the CSFI fuel system, commonly known for developing cracks in the plastic fuel tubes going to the injectors from the metering block. Requires disassembling the intake manifold to service. The bright side is that a standard rail-type FI adaptation of this system was made for marine applications, and it is a near-direct swap with only minor modifications needed. Mostly because the marine engines do not use EGR.
However, it is possible GM may have still used TBI on commercial chassis like this, and that system will resemble a carburetor -- generally trouble free.
And being located in Maryland, it is likely to be badly rusted in areas you can't see. Verify locations and severity of any corrosion underneath, and I highly recommend posting such pics for others here to review, because rust is a major consideration when buying any vehicle from the northern part of the country.
I'm not saying don't buy it, just be aware of these things. The rust is honestly a bigger issue than the fuel system, but if it has CSFI, it is expensive to repair when it has problems.
"Cheese Wagon" <firstname.lastname@example.org>
Former owner - 1989 Ford B700 64-pass Blue Bird (Rest In Peace, Cheese Wagon)