I have some 1/2"-thick steel plate (don't know composition, it isn't
stainless) from which I'd like to make two make two 12"-diameter disks.
Each disk also needs a 3"-diameter center hole, and a pattern of
fairly-precise though holes for bolts. Think of something like a
simple brake disk.

I think I could rough-cut them with my torch and then pay someone with
a lathe to finish the job. They could chuck up on the outer rim as
best they could, cut the center hole, then chuck up on that to finish
the rim and mark a circle for the through holes, then drill them on a
press.

I've also heard of a process called "water cutting", whereby a
high-pressure water stream is used to separate material. I am
concerned about heat warpage if I flame-cut the disks, they must remain
flat and true. Can water-cutting be used on 1/2" steel?

I suppose wire-EDM is also a possibility, but it sounds expensive.

What procedure should I consider, and how much should I expect to pay?

thanks,

Martin