Among other parts, you'll need the boot that connects/seals the swingarm to the engine/trans output.
Still available new OEM from Honda. Online prices are around $20 US.
52101-HA8-680
The 85 had a different system of seals on the brakes compared to the later 86/87. The later years were generally considered better at sealing. I had an 85, and still have an 86. I found that the 85 would get water in the rear-end even after only minor water exposure. Just make it a habit of checking for water in the final drive if you have any water exposure when you ride.
I think you definitely need to make every joint as clean and tight as possible, but even if you do that, the leak path through the rear brake drum is the weak point in my opinion and there isn't a whole lot you can do about it other than keep an eye on it.
- Frank
1984 200ES Big Red
1985 350X (x2)
1986 350X
1986 250SX
1984 Auto-X
1984 ATC70
1985 ATC70