97 is relatively young so it should not have rusted through (hopefully). As an owner of a variety of rusty vehicles over the years, I know that when rust appears up through the paint, its always a bit more extensive than it looks.
Be ruthless in exploring the extent of the rust to try and catch it all. If you find that you are opening up holes in the metal, its not a huge problem because there are various fibreglass or filler kits you can buy to patch these up. Just be wary of trying to rebuild corners of a panel or filling very large gaps - its not usually very successful and you'd be better off seeking a good condition replacement from a breakers yard.
I'd be surprised given corrosion protection these days if there is serious structural rust, but things like rusted through sills or crossmembers etc, underneath will need to be patched professionally with welded-in steel to keep your van road legal. Fibreglass patching on structural areas will be quickly spotted by an NCT or DoE inspector and is dangerous
Handy helps for your work will be a selection of large and small wire brushes, some of which might be fitted to a drill, and rust neutralising treatment or rust neutralising paint. 'Curust' is an old treatment from the 70's I recall and Finnegan's Brown Velvet is a good neutralising paint treatment.
Preserving your work by stripping off any old underseal and reasealing the vehicle is a good idea. Hammerite or smoothrite paint is tough and good looking and isn't hard to match to most other colours, so it can be used extensively on sills or wheelarches which might get a pounding - you can paint it on too and it has a relatively even finish which is perfect for not as noticeable areas such as under doors and bumpers.
If you find that the van is rusted literally all over and you have the budget, go for a complete respray. It depends on how much you want to spend, but it'll help avoid it looking patchy and 'worked on'.
Just be aware that if you change the outside colour in a respray, it won't match that inside unless you strip your van to its shell. Personally, I think its always better to stick to the original colour because it avoids all that hassle and to me, looks a lot better.