Just had a load to do to get my MOT, before I costed the parts I set to with a wire brush and grinder to see if it was worth it. I had to replace rear floor, serious repair around both outer rear seat belt mounts, 6" section on the N/S sill at the rear, O/S/F body mount to sill. Job looked to be a couple of days so made enquireies from local welders, estimates came in at around £500. Checked on cost of parts and a basic welding kit from Halfords, that all came in at £200. Set to buying the parts and a couple of packs of rods and taught myself to weld, all the work took me about 2 weeks, a bit each night and weekends. The whole lot satisfied the MOT tester even though the welding is far from pretty, it has done the job and I'm okay with that. Forgot, the body mount got away with a bit of grinding and a plate to the side.
There is one thing to watch, welding upside down is not only dangerous it nigh on impossible as I found doing the sill because molten metal reacts really well to gravity and when it hits the ground it goes everywhere. Dont' do any welding without the right gear and that includes an apron, might be expensive and look daft but molten metal really hurts. Loads of patience required.
Bite the bullet, it's great fun and another skill I'm glad I've got.