I had to do the same with mine, bought it knowing i had to rebuilod the jacking point.. Started stripping the underseal away with a wire brush in my grinder onlyu to go through lots of metal with it, so i ended up doing:
Rebuilding jacking point
Replacing the entire sill
Replacing the 3/4 floor pan
I then had a piggle the other side and ended up doing:
Rebuilding the jacking point
Replaced 1/2 the sill
Replaced the entire floorpan
Plated over to bung holes
Then one of the arch trims wasnt stuck on quiet right so pulled iut off to find that i needed to repolace an arch...
all in all with the the rear arch, two sills and some shaped floor pan tin, a litre of underseal and some seam seal i think it all cost me £100ish IIRC
My uncle started me off and showed me how to do it, i then did the rest myself so no labour costs..
Good luck on getting it fixed up Shady