Welcome to the world of Earlybay!!
I also have a 1972, which I believe was the year of greatest production, certainly for early bays. My van was my firt camper and I went straight in at the deep end without much knowledge or preperation, but hopefully it seems so far as tho I have gotten away with it (famous last words??).
There is a section on here with a buyers guide: http://forum.earlybay.com/viewforum.php?f=21" onclick="window.open(this.href);return false; its well worth having a read through, there are also several guys on here who would happily source/sell you a van.
Belly pans obviously offer protection to the underside of the van from the wet, but as you have pointed out can hide whats underneath - you should certainly check if the pans are intact or have rust holes, evidence of removal/refitting?
Parts are generally freely available although most years have different parts for different components, tho I believe the '68 is the most difficult to source.
Anyway, take your time with a purchase and make sure you read up plenty before parting with your hard earned! This site is a great place to start, especially the for sale section!!