The process is still yours to decide. But i can give you some food for thought form a more business type perspective to help you along to an answer dont think of it from the perspective of the guy with the helmet think of it for a bit as the guy handing out the invoice
the two biggest drawbacks to TIG are the slow speed and the vulnerability to the gas being blown away. Tig is not a good field repair process compared to stick and mig/fcaw there is a lot more setup in trying to get any given part out of the wind and getting yourself into a position where you can weld the part so setup time is more making the weld more expensive at the outset.
the welding itself is slower and deposits less metal once again making for a longer repair and more hours and more time.
a tig welder job can request a premium for the jobs that cannot be done any other way since you do need more skill to truly tig some parts... but that might not apply to you.
first question is are your customers willing to pay the extra for tig and a tig welder when a mig or arc job will be more than sufficient. You need to be reasonably fast and efficient and not price yourself out.
the other question to ask is how often will aluminum or stainless actually cross your path? tig does excel at multi meals and fast changeovers if you would expect to see a half dozen different metals in a day then that leans for tig but if you need to be ready for stainless or aluminum every few weeks or so then having a spool of the right wires ready for a mig makes way more sense since youre ready for most things instantly and you can handle the occasions with stainless or Al when they crop up.
Its more economical to have a spool of AL or SS handy if you are mainly doing mild steel jobs and depending on the situation you could have a portable wirefeed to do field repairs or use a spoolgun.
Just remember that youre looking at that from a business perspective and as such solid welds as quickly as possible will help you be a more affordable welder and get you more business overall
Just some food for thought from a small business perspective