Mine, a '96 T1N in Arctic white in the UK where we spread rock salt on the roads if it looks like getting near freezing seems to be mostly rusting from the inside out, presumably as a result of condensation running down the insides of panels and pooling along the inside of the sills and bottoms of the doors.
The original (I assume) underseal is doing a great job and I can't see any issues with the chassis rails, except just forward of the forward jacking points, where holes in the top of the rail (for radiator mounts if I recall) line up directly with the rust, so clearly water has run into the rails there. Likewise the floor seems to be in great condition.
As Richard says the seam sealant is cracking on most seams, on the side panels this means the rust extends a bit higher where the seams are, to above the sills. I have just done some sill repairs, almost all of the drivers side! Cut the rot out and made inner sill piecemeal last weekend, made new outer sill piecemeal this weekend (including up onto the side in may of the seams). I wasn't particularly tidy with it as the sills are pretty well out of sight most of the time, I just needed there to be no sharp edges in preparation for MOT test so I did it quickly rather than neatly. There is a shoulder where the sills transition to the sides and my fabrication is pretty poor at replicating those so I may well buy some repair panels in the future and re-do those areas.
I have managed to prime the new sills, and have used brushable seam sealer on the inside exterior tonight, unfortunately it started raining afterwards so I haven't yet seen how it looks dried, got a feeling it might crack a bit and need some touching up - this stuff makes an ugly finish so don't use on outer panels, for there I need to use the sealant in a tube which I can smooth down whilst wet and which will look OK once overpainted.
My plan is to apply underseal (schutz) over the seam sealer, both products should remain flexible, only worry is I haven't had a chance to test compatibility. I will then use cavity wax to coat the interiors of the sills and hopefully protect from water running down the inside of the panels - there are plenty of plastic hatches to pull off to gain access to the inside of the sills (rust was below the level of these).
This is not the only way to do it, some people prefer stonechip over underseal, and some people will spray a coat of wax over the top of whatever else they use as extra protection against salt. There is no shortage of confusing rival products out there to help you proof your van or car!
Apart from the sills the lower parts of the doors rust out (same reason, although there is a recent thread about window seals, I noticed after that that mine don't seal either), and the front wings and wheel arches - wings due to the drain above the bonnet sending water down the inside of the wing where it is just primed. The wings are easiest to change, there is a bit of sealant to cut but aside from that it's just bolted up, and replacement wings are easy to find (mine are actually second hand ones that were in good condition, but I cleaned up and painted to be certain). Front wheel arches are not too bad but need to be cut out and replacements welded in - they are complex in shape and have a lot of spot welds to drill out, but actually with a bit of care over alignment I found they passenger side went fairly well (not done drivers side yet).
Door skins are available (partial so you will have a seam somewhere) but the frame rusts out too. Only done the passenger door so far and I was able to rebuild the frame using flat bar - not perfect but holds the skin in the right place. The slider skin is currently completely separated along the bottom, i have taped it for now and hope it will be acceptable for the MOT - I do have the replacement skin but have not had time to fit it, and the frame is going to take more rebuilding than the front door did.
I do have rust in other places but the bulk of it is sills and doors.
For your purposes, if buying a van that has spent it's whole life in California and is still pretty immaculate - I reckon you should think about cavity wax for inside the sills,chassis rails and door skin-coat as necessary.