If it were me, I'd get a replacement floor.
Have a look at the outer floor sections too, poke and prod and see what condition they are in.
Regarding the rear cross member, I'd chop the whole lot out and put a new one in to be honest. If it's that bad, the chances are the rest of the metal will be **** quality, and that means welding to it will be a pig. If you can get a decent arc going it will blow through all the thin crispy bits, I'd change the whole rear section myself, and I'm a professional welder by trade.
The good news is, if someone asked me I'd say it's repairable (taking on-board you saying the sills and the rest of the chassis etc are fine), but the cost of doing the job would come pretty close to the vehicle's value, I'd cost the job at about £500, personally.
What I don't really agree is that Discoverys for £500 with 12months MOT that aren't going to have very similar rust issues themselves are ten a penny though.