It could be a very good thing to get into...especially if you are in an area that has a lack of machine shops around, or if the nearest one is an hour or more away.
But its the quality of work that will ensure success!! That and customer service/relations.
I have a friend who has purchased some machines, mostly the cylinder head equipment. He's started out in his garage, and he's already swamped with work!!
He's done some top level stuff (like alcohol funny car heads & the like), and does a very good job. He's very thorough and does'nt let anything go out the door that has'nt been double or triple checked!!
So using his example, you would be wise to start off the same way. Get the valve & seat machine, a surfacer, a lathe and a mill. Get all the tooling for the required job, like a valve spring pressure tester for example.
A good cleaning system and blast cabinet, magnaflux unit & more.
You'd be off to a good start there.
As the world turns...I get more dizzy!