Regarding the actual Discovery and the welding needed that doesn't sound too bad, well it's bad but normal!
I did about all those things to a Discovery I am currently rebuilding.
A tip on the sills, don't buy the panels, buy some (I think 100x60mm) box section for the sills. £115 all up for the panels, £30 for a piece of box section and some offcuts for repair sections.
Remove the trim panel, remove the door seals, and lift the carpet back or remove and you will see the rotten stuff, just below the lip that the door seal fits over, cut right along this so you are cutting just below the floor panel.
Cut down at the door pillar, right across the underside and along the back of the sill leaving a small section remaining sticking down from the floor pan.
Once you have fully cleared out all the rusted metal, cleaned up and painted the area, offer up your piece of box section and weld it in. I capped mine before I fitted it.
I did the drivers side with the repair sections, and the passengers side with the box section method, it's a lot easier, and stronger and means you don't need to worry about jigging the door pillar lining things up the same as your not removing as much metal.
If the floor needs any repairs you can then cut them out and weld in new pieces.
http://www.landyzone.co.uk/lz/f8/discovery-sill-replacement-103369-3.html
or here on a RR - same job.
Replacing Range Rover Sill (s) - LR4x4 - The Land Rover Forum
I'd buy the rear body crossmember and boot floor panels - they are worth it.
If you need new rear seatbelt eyebolts and clevis pins I can get them for you at about 1/2 the price, just save the little metal bracket; although I made new ones by cutting 3mm box section to the same shape and size as the bracket, drilling it for the clevis pin and welding a 7/16both bolt to the underside.
Brand new seat belt mounts for about £15!