Welder advice - where to start

MIG welding is the easiest to learn, and with a little practice and knowledge, you can weld sheetmetal with a MIG. The technique involves many small stitch welds, done by moving all around the seam and not welding in any one place too long. Most shops can weld pretty much anything with a MIG welder and some experience. Floor pans are much easier to install with a MIG than any other welding process. If you can only afford one welder, get a MIG, some solid core wire, a bottle of 75/25 argon/Co2 and practice with it. Good welds are all about practice and learning.

TIG welding is somewhat more difficult to learn, but produces cleaner, more precise welds and is generally the tool of choice for thin sheetmetal, because it causes less warpage. Plus the finished welds are more easily worked because they are less brittle when cooled fully. Setting up the material to be welded is completely different from one type of welding to another, so you need to know what to do for the process you are going to use. (MIG welding sheetmetal requires a small gap between the pieces, TIG welding requires them to fit tightly together at the seam.) Gas mixtures are also different for each process.

My advice would be to buy a MIG, practice with it, and see if you can accomplish everything you want to do with it. Maybe take a local welding class if possible, because there really is a lot to learn. Maybe that experience will tell you if a TIG system is worthwhile.