Just thought i'd update on my plan to address.
i've got some Sikaflex 227, rust converter, some rust proof primer, and a (hopefully) matching paint spray can. Went for DSD42 DupliColour.
Plan is to wire wheel all effected areas back to bare metal, to assess damage.
If it's very deep I might be in the shit- but if it's only mild/moderate, i'll be applying rust converter, wiping clean, seam sealing, priming and painting all effected areas, are stripping the rust off using drill + wire wheel
I also plan to soon paint the wheel arches or even full underbody to prevent any more water entering, as i'm assuming this is where 95% of the issue arises from, especially without guard liners.
Will update once i've done this. Planning on doing a bunch of other general resto work (headlight open + clean etc) so hopefully I can update within a few days.