this pic is actually really promising:
no rust around the rear seam where the boot floor meets the rear panel
a good start. very very common place to rust.
with your boot floor, i have to be honest and say that your best off getting a panel piece from rare spares. even though it wont fit and itll cost you a tonne, its the best way to get it RWC. some testers will have kitten if you try and repair a hole that big.
now, what i want you to do: pull that back windscreen out and run over the bottom section with a wire wheel on the angle grinder. from the looks of the rear panel, you may find there is little/no rust. fingers crossed. the back window is pretty easy to get out. from the inside of the car: just lift a corner of the seal with a blade screwdriver, put your feet on the screen (weird position, but it works) press with your feet while pushing the seal out. the rear window flexes quite a bit so dont stress too much, but also: dont go overboard cos' they WILL crack of you go too mental.
keep the pics comain mate.
oh also, rip off the boot seal and show us a pic of the two corners towards the front of the car. (please)