What can I fab?

Got a 93 200Tdi Disco with obvious rot issues, and a few questions as I'm preparing to sort it out for MOT, and hopefully for good.

Sills : If I cut the bottom off and weld a 3mm bar, will that make sense? Seems more doable than removing the whole sill piece. I've read about a box section replacement, is that legal?

Rear sill : If it's got holes in, will it fail? And can I replace the rear body mounts with a decent weight fab?

Boot floor : Can I replace the crossmembers with 25 x 50 mm channel and seal a sheet on top? How important is the tank inspection cover, and are there any special requirements for the seatbelt holes?

Rear arch inside door : Not too bad, but can it fail if it has holes?

Thanks

Been reading on here (Chrisw8's thread is great), and so far I've managed to work out :
- Sills : Just cut away the outer and seam weld in the box section. Seal up the ends of the new sill, treat and seal in-between old and new. Patch anything connected to this. If the MOT man can hit it with a hammer and it sounds like metal, it's good.
- Rear arch : Top is near seatbelt mount, so patch it good if any doubt.
- Boot floor : Has seatbelt mounts and I want to carry loads in here, so needs to be good, otherwise not that important. Using channel for floor supports will be a hundred times stronger, but will require a bit of clever cutting for fuel lines. If I take the rear seats out I can drive it without the floor if I need to.

Still not quite sure about :
- Sills : They're really not that bad, just a few holes in the bottom, but it's been patched proper ugly. If I just cut the whole bottom off does it still count as a patch or have I ruined everything?
- Boot floor : Do I need to use a pattern part? Do I need to seam weld this? Ideally I'd cut back to good (not far), weld or bolt in supports, fit a sheet, drill it for seatbelts, seal and tack/rivet/bolt around overlap. If I lose the lip I can probably do this without removing the tank If I have to.
- Rear sill : Frame xmember is good, cut and patch the rear body will do, but one of the body mounts looks a bit sad. Treat it, patch it, or replace it? I presume it's down to the hammer again.

Really sorry if it seems like I'm asking questions which have already been answered, but I need to be double-plus-sure about exactly what I'm doing before I start on any of it since it's my daily driver and I can't have it off the road for more than a couple of days at a time.
Any advice would be greatly appreciated.

You are asking: Should I Bodge and will you all condone this? Or shall I do it properly like a professional? The answer is really only one you can answer! Personally I would never Bodge because thats normally a temporary repair AND you'll get no compliments from other Landy owners lol

I don't seem to be much good at asking for advice
It's not so much a matter of condone as one of what's legally acceptable. All I know is "no rust holes in anything structural or within a foot of seatbelt mounts", and I don't want any surprises.
I don't want to bodge any of it if I can help it, but I don't have the money to put it in a shop and have it all put back to original, and, given its weaknesses, would that be any good anyway? Some might say that an upgrade is a bodge by another name, but I'm looking for a workhorse, not a concourse winner.
I suppose what I'm trying to ask is : If I can fab a part that's stronger than the original for a fraction of the cost, does anyone know any just cause or wossname?