I can't speak for the 1200 or 600, but I changed the seals on my 400 like three times within 6 months of each other. That being said you probably won't be listening to my advice too much anyway but I will offer just in case.
Definitly do both seals, the reason being so you won't blow the other one and have to replace it later. Plus you don't know the fork oil level or condition on the other side unless you replace the oil.
I didn't replace the dust covers anytime I did mine just because they don't seem to do much.
The 400 manual called to replace everything inside the fork when the seals are replaced, inner and outer friction metal, retaining clips for the seals, dust covers, washers and whatever I might have forgot to mention. I didn't replace all of this the first two times but figured I would give it a try the third time (minues the dust seals). Didn't seem to matter because even after the third time they started leaking again. I think I had problems with the fork tubes and sold the bike with that problem.
1.5" PVC is most likely for putting the new seals on, you will want to slide them on and place the old ones on top, in a mirror image (so the tops of each seal are touching), and use the PVC pipe to drive the new seal in place.
I had the best luck with the seal replacement the first time I did it. I used bike master seals and only had to redo it because one blew, I think that was due to a weak retaining clip. The other times the seals just started leaking but they were not bike masters.
There was also a trick with a straw to get the oil level correct in each fork. If you want I will look for the thread for that and post it in here.