Andrew;
Azruss has some very good points to keep in mind. I recently did a much less ambitious rust repair than you and I was challenged by some of the more technical aspects. It is definitely doable but here are some of the "if I had to do it agains" that I would change.
I purchased an inexpensive Flux core MIG welder. The wire feed and flux wire worked fine, but my welder only had a high and low heat setting. I found that the low wasn't low enough to prevent all the "blow through" hole in my repairs. Make sure your welder has a variable heat setting, not just high and low. A more skilled welder would have more tricks to deal with holes and blow throughs but it does take alot of practice to get to that point. My three pieces of advice are: 1)tight fits between old and new metal - no gaps 2)use a "heat sink" like a piece of copper behind the sheet metal to help cool the metal being welded 3)lots of small tack welds
Remember to treat with rust converters etc. on those inside panels before you close them up. You never want to have to do this job again
Lots of patiece! Once you have a good solid piece of metal in place and some good body filler everything will start to look much better!
