Dentino3

I am up to my elbows in rust right now. i just bought a 95 Cherokee Sport. Due to the fact that my powers of observation are suspect, it was raining, it was night or maybe I was just too drunk, I bought a car without crawling all over it. I now know that a major portion of both sides of the floors are rotted and both rockers have a 5" long hole starting from the rear and going forward. I'm putting it up on a lift in the morning to see just how bad it actually is. But so far the doors look good and nothing above the rockers shows rust. I want to fix this by welding in new floors and rockers. The question is, how far do I go? Can I stop cutting out metal when I can't punch through it with a screwdriver? Should I sand blast off the rust inside the rockers before welding on new pieces or is it enough to scale off the loose stuff and treat it with a product like por-15 after welding on the new sections?

Any guess on what I should be spending for the welding? Hourly charges for an experienced welder? How much experience should I be looking for when I look for a welder. Most of the welding needed will be hidden (as long as I keep the shiny side of the car up) so is replacing the floors something I could try to tackle myself? I realize that costs vary by region, I'm sitting in central Illinois right now, but any guidance would help.

All suggestions great fully accepted.