I noticed a "crack" in the undercoat when I was having a tire changed. Turns out that my passenger side floor is rusted at the seam between the rocker panel and the floorpan. About 3" of metal are "crunchy" along the seam between driver and passenger seats. It seems to have rusted from the outside in, not from the inside due to water intrusion.

Questions:
(1) Can anyone recommend a body shop either near NYC or near Morristown, NJ that will undertake this sort of repair? The previous guy that did this type of thing for me seems to no longer be in business.
(2) What should be the cost of fixing this if I grind away the bad metal myself?
(3) The rusty strip is within about 1" of the outboard rear suspension mounting bolt. The metal around the bolt, in front of it, and inboard of it is good (passes the screwdriver-and-hammer test). Should this area be reinforced in some way, perhaps with an L-section welded to the rear seat box and to the captive nut in front of the rear seat?
(4) The rusty area of the floor is basically flat. Has anyone had any luck buying a MIG welder and doing the repair himself? I have a space to work in outside the city, so this won't be a problem.