Bluehawall good question and like KnC I am intrigued to the reason behind the post.
My answer would be that it would depend on your location. If you are not from an aviation engineering background a lot of money can be wasted buying equipment / spares that are rarely used. Equipment hire as required is a better option than trying to equip to cover all possibilities.
Do not know your location, but if Europe then there is the dreaded EASA. You would need a full understand of EASA OPS, EASA PART M and EASA PART 145 for starters. Your company would need to get EASA approval to maintain aircraft and what specific types of aircraft and scope of work.
Then there is your customers aircraft. They would have to have agreements in place with you based on the maintance procedure drawn up as part of their Continued Airworthiness Programme.
Appointing an experienced engineer would be a priority to help get through this minefield.
Tooling and spares will only play a small part of setting up a line maintenance operation.
In this present climate would you get the customers to make your business viable.
Any way Good Luck and I hope you succeed.