You could spend a lot of money on parts only to find the person who inspects your aircraft for the next ARC refuses to sign it off. It happens.
One problem you face is that EASA will not allow you to validate an FAA STC. In my experience, EASA will only accept an application from the firm that already holds the FAA STC.
If you are new to ownership, you would be better going to a good maintenance outfit and asking them for help.