Your description is a pretty good account of my project one there Craig
Rust areas to look at are upper bulkhead corners ( dash out, harder on facelifts models as the frame is welded in).
Upper front valance is very expensive ( I've a couple of outer end repair panels).
The triangular piece of inner sill ( rear end) which is actually outside can be awkward to repair due to restricted access caused by chassis rail & rear arms.
Easy with sill removed.
Scuttle corners can rot bad & be full of filler. Patch panels are avl.
Kick panels can rot badly I can supply repair pieces or full panels at times.
Spare wheel wells rot but if not fussy, I've cut them out of other cars sierra & escorts used to be ideal.
Front corners of roof skin are starting to show rot nowadays same for rear screen scuttle corners no repair pieces ever avl for these two.
Front cradle lower mounts can rot badly now repair pieces never avl ( pre & post facelift have a different size mounting hole) MK4 is I think same as facelift, Mk5 the panel is taller can be trimmed to fit.
I have used prefacelift ones & a pattern avl for both MK3 versions.
Most other rot is of the what you see is what you get variety.
The bulkhead can be the deal breaker depending on your skill level/patience.
I'm the patron saint of lost causes when it comes to this stuff lol.
Some prefacelift parts are unique/expensive switches etc.
Some stuff can be swapped for complete later units ( wiper mechanism, steering column).
Couple of worthwhile mods are ohc efi, 5 speed box.
After that the world is your lobster I've done all sorts from convenience type stuff Lecky windows, seats(Inc heated), folding rear seats (saloon) uprated brakes Inc rear discs., through to 4wd,,super or turbocharged to V8.
Always happy to help with parts,hints,tips etc if/when I can.