What is your budget? What is your expectation for return on your time?
If your budget is very low, you can pick up EM's for astonishingly little money, and people who are familiar with EM pinballs are always in demand. You would build a skill that is in great demand.
If you have a better budget, go ahead and buy a shopped machine and a playfield and do a cabinet restoration and a playfield swap. This is a lot of joy because when you are done you will have produced something greater than the game was originally meant to be. You are only going to do a limited number of restoration projects in your lifetime. Might as well make every one a timeless classic work of art.
If you expect to get money for your time... well, you can do that but it takes a lot more discipline.
When doing restoration you live in the question of 'how far do I go'.
To get your money out of your project you'll have to choose a machine that is mostly good, and improve it, but not go to the highest standard. Quality where it counts instead of quality above all.
If you want a long-term project, lots of sweat and time but not much hard dollars invested, think about your machine purchase in those lines. What can I do with just time and effort? This will guide you to machines that might have been close to a fire, or are insanely filthy but intact.
If you want to pay yourself for your training, you might have more in the cost of the machine and the parts and the tools than you will ever sell the machine for, but you have improved your ability to be more capable in the future. These machines would be ones that parts are available but you'll need a lot of them, and there will be lots of ongoing costs in getting your refurbishment project started. It won't matter, because you are investing in yourself instead of demanding your hobby be cash self-sufficient.
In any event, I wish you well, and along the way, learn the skills to do home service calls.
The single biggest problem in this hobby is the lack of people who will go to your home and fix a machine, so think about developing the skills to be one of the guys whose hobby supports others where they need it... in their home.