To take the contrary viewpoint, TIG is NOT difficult. If you can master Oxy acetylene you can easily master TIG. This is an excellent process for making very solid, strong, neat, good looking welds. I personally own Miller equipment and have welded hundreds, if not thousands, of mild steel, stainless steel, 4130, aluminium, and magnesium parts over the years with great success. While I'm happy with Miller, Lincoln or almost any other name brand will give excellent results. The more "features" you buy, the easier to use, or the more versatile the machine. Talk to a knowledgable distributor for advice on the features beneficial to your desired use.
Although not difficult, there is a learning curve and as Ross mentioned, practice, practice, practice, then practice some more. I personally find "burn through" as mentioned by Paul, much less likely and easier to control with TIG as opposed to other processes. A talented TIG welder with the right equipment can weld aluminium foil together. (I can't but have seen it demonstrated)
The decision as to whether the effort to learn is worth the satisfaction of mastering another skill and whether the return on that investment is justified is yours alone to make.
Paying someone who already has the skill to manufacture the one or two fuel tanks you need will undoubtedly be less expensive then acquiring the equipment and skills to do them yourself, but at the same time, for some, the learning is just as important as the end result. As I said above, your decision.