It is not cheap, fast or easy to do a stainless shell.
I am in the process of setting up to do a shell or two for my car. While I have a TIG welder and what seems to be the right filler rods, I bought a pretty decent shell with no real cracks.
Welding will require a good TIG welder and an experienced welder. I am not a great or very experienced welder, but I try anything. My limited trying has found it easy to mess up a shell welding. Fortunately there are many shells that are bad enough they make good practice. I will add that if you are having troubles seeing stuff close cause your eyes are getting old then look for some cheaters at the welding store. The cheaters are magnifing lenses that fit in your helmet. They really improved my welding.
There are videos on Youtube on fixing stainless trim. That would be start.
You need a number of hammer and dollies and lots of sandpaper. Eastwood sells some trim hammers, but you can make do. Look at what the hammers look like to get an idea of what you need.
Once you get the surface leveled. Then you start sanding. and sanding and sanding and sanding........
You need to guess the right courser grit paper that will level the areas you worked. Then move up to a grit that will take out existing scratches. When you know you have cut down below the course scratches then you start with the next finer grit paper. Hours later you are ready for buffing. You only want to buy name brand paper. Packs of 50 3M silicon carbide are are about $45 each.
You will need a decent buffing wheel. I suggest that you buy a kit of wheels that are correct for your buffer. Tarheelparts.com sells what you need and will help you buy the right stuff.
Then you need to know how to buff safely. Keep in mind the buffer can catch the shell and rip it out of your hands if you are lucky, it can also take your hands with it.
For around $300 to $500 ( those are numbers I have heard) you can get a decent shell buffed out and ready to mount.
I suggest you start with some head light shells. They are safer to learn how to buff.