I'm working around the mini. The rear subframe has rusted out at the corners lay flat on themselves. I took a squarish section  out and welded in stainless piece with a water exit.  The rest of the subframe is still good enough to keep.

There was a heat shroud because of the catalytic converter, held on with six studs. That  caused a rust out area about 9 to 12 square inches  6 different places at the tunnel. 

I'm going down this retched rusty bunny hole right now. I just started to look and dig deeper into mine after it was stripped down to bare metal before it gets blasted. Almost thinking of not bothering getting it blasted now.
Now here's a phrase you don't hear everyday: "She looked and felt better with all the bits on. Now that she's naked, it frightens me." 
I have everything wrong with her, that's on Dan's list, minus the battery box and wheel well. But raise you a rotted boot floor corner under the tail light and a rotted out lower bulkhead/firewall section (that the lower part of the front subframe mounts to) where it met the rotted front floor sections. 
The only metal sections that are really good that only need a light sanding are the bonnet and boot lids, and the roof. 
Took a heat gun and a putty knife and lifted out all the sound deadening out to see how much issues there was going to be under all that. Surprised to see none on the whole middle-to-rear floor. But shocked to see how many stamped/drilled holes there are in the floor. 
Makes me wonder how long it's going to be before it all starts to come back after I blast, sand, paint and undercoat. 
Good luck out there. I'd say that pretty much anyone that did or doing a full build, blew their budget on repairs and replacement parts. 

On my previous mini ("Barney the Floorless Wonder Mini") I found 2 or 3 places that seemed impervious to rust.

The windscreen , the plastic gear knob and the tyre treads.

Most everything else seemed like fair game !

I kid of course, but after hauling that barn find home from Ontario 1100kms away and seeing just how bad it was in person I learned a few things:

-Don't send your brother to look at a damp barn find Mini when the temp is -20C...."everything is solid" (he said) (substitute the word frozen for solid and he would've been correct) (@ +20C everything was stinky mush!)

-So many things can seem OK visually, but are seized rock solid

-Mice and Rats eat wiring.  They seem particularly attracted to the Lucas variety

-Don't buy any more 'barn finds' (if I want to live to see my next birthday). 

-Not all barns are nice and warm and dry.

YMMV

When looking at a car, I sit the back seat and pull up the companion box trim.

Under neath should be the inner sill and floor sections welded to a front-to-back closing stiffener plate.

Often missing and normally not repaired as its so difficult to get at.

But a sign of the quality of work that gone into repairing the car.

How a mini rusts, and how!

Wouldn't it be simpler just to list the parts that don't rust??

Here's my list for that:

Front windscreen
Passenger and driver side windows
Rear windscreen
Tires
Upholstery/headliner
Glass on mirrors

Besides everything you listed, the 90's mini rust like crazy. I have to add:

Seam under headlight,

rusting under front wings,

the areas where the useless flat "U" at each corner of the floor pan, rear wheel arches.

Rover sprayed under coating under the body which traps the water and sealed over all the drain plugs.

The lower front overlap seam at the wheel well.

The seam at the  rear bulk head and floor pan.

Sometimes at night you can turn off the lights in the garage and hear it rust!  Tin worms never sleep......

Floors will be pretty early on if there is a windscreen and/or heater leak.
Petrol tanks pretty quick in a stored car.

Thinking about the thread "Rust and Restoration" and that they all have rust, It occurred to me that a sequential/progressive list of where a Mini will rust would be useful. Here's from my experience:
- outer sills (rocker panels)
- inner sills
- A-panels
- rear subframe
- quarter panel ahead of rear wheel
- rear valance below bumper
- battery box
- spare tire well
- door bottoms
- cowl below windshield
- windshield A-pillars
- front wings around headlights and at seam with A-panels

Please add your own observations - my experience hasn't extended to roof rust... yet!