What grade? What ever you get, watch the temper, especially if your milling plate which is readily available 1/2 hard, not so easy to find annealed, atleast with my local suppliers.
What process? Milling, turning?
Coolant, if any, depends on process and tooling.
Tooling depends on process, machine ridgidity, grade of stainless, spindle horsepower and RPM. So on and so forth.
You should probably post as many details as possible so people can argue over the best answer.