I used to be a machinist for a very successful speed shop. I can give you the same advice I gave to friends back then.

Don't pay for anything in full up front. Only pay the minimum to get the work going and be sure to make it clear that you will pay the rest of the money when the job is done and you pick up the work and or parts.

That motivates the owner to get the job done in a more reasonable time frame.

You wouldn't believe the number of shops that run on other peoples money while putting customers off so they can live off the money while not delivering product or services.