I am also going to be doing some rust repair on my E39, mostly rocker panel and trunk.
I will be using 22 gauge sheet metal from here: http://www.tscstores.com/12-X-24-22-...EET-P1952.aspx
Also, if you can convert you flux cored to MIG do it! I just converted from flux cored to MIG, and the quality of the welds is SOOOO much nicer. AND it was much harder to blow a hole in the sheet metal.
If you are buying anything from www.eastwood.com
, get it here: http://hawkesauto.com/
They use the US price and only add exchange.
I just picked up this HVLP gun:
and am thinking of painting my whole car....
Good luck on your rust work!