Hi Mate,
how is the car generally? I think you would be as well to get it through the MOT as it doesnt seem like an awful lot to do really! How much are you going to have to spend out to replace it?
As regards to the welding it seems a bit much that the car has been failed for corrosion on the retest that wasnt noticed before! I know that you say the guys that done the welding had to move the side skirts but thats not your fault! I would be back down there and have a right moan and a groan about it see if you can sort something out.
Rear calipers arent the end of the world most of the local motor factors can supply these exchange and they are not too expensive. I done a pair on mine last year with new discs and pads and was under £200 ( trade) made a world of difference to the handbrake I can tell you!
Where is the power steering leaking from? Adjust the CO with the screw on the ecu, your local friendly garage should be able to do this using an exhaust gas analyser.

CheeRS, Mike.