Also, if you haven't already, go browsing at the Sprinter Store:
www.sprinterstore.com.
They have many useful Sprinter goodies, including information about the suspension modifications you might want to make. The standard mod for a T1N is Koni shocks and a heavy rear antisway bar; the investment may or may not be worth it, depending on how you drive and how piggy your van currently handles.
As for the rust issue, Sprinters weren't the best, but the more common spots were around roof seams and, especially problematic, the windshield. There's a good chance the chemicals made your floor situation worse.
Roof racks are all around, but the most easily-sourced ones look/are pretty vocational. You can, however, source Thule high-roof T1N roof bar perches from the UK, which would then give you access to any cross bar/roof rack setup you wanted. They'd look like this:
More pictures of the rack setup are here:
http://www.expeditionportal.com/forum/threads/19224.
It's good that you are excited. Sprinters have many advantages as an overlanding truck. Best of luck.