It would appear as if you got yourself a pretty nice bus!
In regards to the power package, that is about as rock solid as they get. I would definitely invest in a deeper transmission pan with a bung hold for a temperature gauge. Fully loaded on a big hill could toast your transmission if you aren't careful. And depending upon how good the exhaust system is I would seriously consider putting on a new one that is 2.3" or 3" with a cold air intake system. Getting that engine to breathe better will warm it up a lot.
I couldn't tell if you have the beam axle or the IFS front suspension. The beam axle is better because the springs held up better over time than the coils did. If you have the IFS suspension consider adding the helper air springs that go inside of the coils. It will help a lot with the side to side issues those buses had if they had the IFS. Adding helper springs in the rear will not only help with the side to side issues but can smooth out the ride a lot as well. https://www.airliftcompany.com/
When you drive your bus really learn to use your mirrors. You will soon discover that you have a HUGE blindspot on the driver's side of the bus. When you look out your driver's window to the left all you will see in your peripheral vision is the part of the bus that is sticking out. Learn to use your mirrors, particularly the fish eye mirror to check our blindspot. It isn't hard to miss something the size of a semi-truck if you really look and check that blindspot.
When it comes time to get new tires it would be smart to start looking several weeks in advance. The 8x19.5 tires are not a common size. As a consequence they are only made a few weeks every year. In other words, they are not always ready to purchase off the shelf. It also means they are not cheap.
Do NOT try to use recaps in the rear or a heavy traction tread pattern. The best tires are highway rib tires. Any sort of block tread pattern tends to wiggle under a P-30 chassis and can produce some odd handling characteristics.
Good luck and happy trails to you!