what do peeps suggest would be the best way to stop the rot moving elsewhere?
Dry everything out thoroughly - e.g. on blocks and pull out all the plugs, scrape out muck where you can (just about everywhere) and after a good dry week or so, inject Waxoyl into closed sections.
Anything you can see is a bit rusty or on the way but not critical (yet) > good heavy wire brush to remove dirt and rust and paint Eastwoods Rust Encapuslator on it - it works. £25/US quart or spray can (£16) inc VAT - try starting with the straight paint, its quite thick, decant into smaller tin to keep original contaminant free... throw cheap brushes away (£1 shop type, some long handled ones useful) - not in enclosed area - if the smell is too much, get the hell out of it for 5 minutes! Can spray into enclosed sections, or difficult areas. Can be top-coated for apperance with Eastwoods Chassis Black or almost any hard finish paint, but can be got in black, silver or red so there ya go!
If you can get good access, do the Eastwoods first, then Waxoyl on enclosed sections. I'm impressed with it, but there'll always be others who haven't used it recommending something else... but if it's a quick paint over, even if you can't get all the rust off easily, one coat, it's the biz.
Eastwoods from
http://www.frost.co.uk
If it's structural cut it all out back to good metal (a good twistknot wire brush (£10) on a 110mm angle grinder is wicked for this, exposes anything that isn't solid) - then get a similar (.8mm) steel repair plate welded in - butting up is best rather than overlapping on surface body panels.
If there's signs of rust bubbles along the seams, then start behind the panel (where the trouble really is!), heavy wire brush the flange area to clean thoroughly and Rust Encap (very thick coat into the gaps, don't play with it), then rip out the seam sealer bodyside and clean back to where the pitting stops on the panels (maybe 1" away or more) - use a scriber or specailly ground hacksaw blade or screwdriver - I RustEncap deep into the seam and surrounds, but others prime and use Tigerseal or a good seam sealer - it'll come gain though!
Also, note what has rusted and why - nearly always it'll be closed sections that can't breathe or water getting in that can't get out, I always like to design a drain just in case, but that has to be rustproofed too, so think it through - i.e. stop underbody splashes getting in, but let it drain out.
Certain areas can be protected from splash/grit/stones abrasion and rust using corrugated plastic (those old Estate Agent signs left in the street) stuck loosely with RTV or similar - they are easy to fabricate (big scissors/stanley knife), jam in and fix to stop a vulnerable area being hammered by stones/water etc. Always allow a draining gap in case water gets behind - a good area to protect is behind the rear wheel, the snorkel boxes either side, they always seem to go on the front bottom corners, maybe all along the engine flitch panel/chassis rail join, clean and fill that area with sealant to stop water sitting there. Drill the sentre of the snorkel boxes to drain (1/4" min) and protect from below with a small plate... if you've got a DJ with electrics in there, vital it's dry and well aired. Ditto plastic above the wheels under the seat, under the front door step treads etc. but never fit it tight up to a another panel, jam the edges on width and stand it off.
... you did say you wanted a project for the winter?