Just a couple of words of advice-
When you do test your tank with pressurized air, you only need around 4-5 psi. You can over-pressurize a tank real easy. If it will hold 4 lbs without showing any bubbling when a soapy solution is sprayed on the seams, you should be good to go.
Stainless makes a nice tank, but thin stainless really welds nicest with a TIG, although you can mig it OK with 308 s/s wire. Aluminum makes a good tank too, but again you almost need to tig that.
Eastwood sells a sealer (part #10565ZP) that would prevent rust in a mild steel tank. I'm sure there are others out on the market...check at your local auto parts supplier.
Depending on the size, you may or may not need a baffle. If you do put one in, don't underestimate the forces involved with the dynamics of the gas inside the tank. In other words, weld it in good and solid...far too often people feel that just tacking it in is enough, and it ends up breaking loose and rattling around inside the tank.
As for plastics, they are a bit more specialized to work with. We weld polypropylene quite a bit, but it is not good for fuel applications. If you do opt for plastic, make sure that the polymer you are working with is compatable with fuels.
Get your sending unit beforehand so you have the dimensions for the hole and mountings before you put the tank together. You may want to weld a reinforcing backing plate where this will go, and tap the screw holes into that. Insert the unit with the gasket when pressuring up to test. Don't forget to put a removable drain plug fitting in, along with you fuel line and return line fittings.
Good luck-
tjs