Welcome! I'm new to the community as well. As a result, others may be able to offer better insight. My wife and I bought a 1985 345 recently. The information available here is great, and the people are better.
What I've learned so far is all these old coach's need TLC (we all have a repair list), and unexpected events are part of the adventure.
I found this posting helpful with things to fix and it discusses others potential issues I hadn't even thought about.
Regarding where to get parts, my experience is - it depends on the part. Although I had heard of them, I finally checked out Camping World's website yesterday. They appear to have quite a few choices for the "house" part of the motorhome. Parts for the P30 chassis is a different matter.
I recommend, if you haven't already, getting familiar with and/or finding a good P30 chassis mechanic. These machines are expensive to tow. You want the chassis to be as reliable as you can (afford to) make it.
Looks like you're missing a windshield wiper arm. We needed washer nozzles and found them here https://www.tacbusparts.com/Other-It.../Spray-Nozzles
. They offer arms too, but I don't know which would fit your year. The arms in our 345 are made by Trico.