[Tercero]

I have a 1997 Saturn SL1 with approximately 136,000 km on it. I took it into a quick oil change service place today, and the service manager asked me to come over and have a look. The frame rails on the driver side are starting to rot out. It's actually come away in some places, allowing the motor to sag by 1 or 2 inches on that side. I was floored. The rest of the car is in excellent shape, and I've kept really good care of it.
I brought it home, put it up on ramps and slid the dolly underneath, and it really is rotting. I could put a screw driver through in some places on the unibody.
So, my questions are,

Can I cut out the rot on the frame, and weld in a new piece, or should I leave the stuff in place, clean off some nice areas on the frame to get a good weld, and weld pieces to the frame.. I have a good Lincoln SP-175 (240v). I'm just a hobby welder, and have only used the welder without gas. Should I weld with Co2 or Argon mix or just shielded wire, and what wire should I use. Can I damage any of the elecronics in the motor if I weld with mig? What precautions should I take? Any help, hints, ideas or advice is appreciated. I'd take it to someone in town, but, apparently no one locally repairs frames. Only sheet metal. I don't want to junk my car.