To be honest you need to have someone physically look at the car to assess it rather than us throwing guesses at you.
Like most things mini, what you see is the tip of the iceberg.
I would say you will need complete inner arches rather than repairs, the door steps, replace completely, there will be more under them.
Complete inner sills will mean going under the companion boxes and to the heelboard. Not the easiest of jobs to do especially as the companion bins need to come out to do this job properly. As the bottom of the quarters are rusty this will also help. The inner arches are easy to do with the rear quarters off, this will also assist in doing the inner sill.!!!!
The boot floor repairs. Now i would be looking at the state of the boot floor on the underside as well as inside. If its crusty at all underneath then realistically a complete boot floor would be the way forward. You are replacing the rear valance and boot hing panel anyway so it will all be apart.
So it all sounds a bit daunting, but if you are thinking of keeping the car long term then its worth doing properly.
Don't forget to use your TMF+ membership discount as this will help with panel purchasing.
Also what i have described is what i have had to do on two full restos and another resto without a boot floor replacement. Hindsight being a wonderful thing i should have change that on that particular car as well.
Look under my signature at Project Paddy and also look up Wills Mini Special. That will show you everything that will need to be done to your car and more.