Can you define “European cars”
Depending on what you are looking for I might be able to assist you.

VAG, BMW, Mercedes petrol/diesel engines mostly if defined that way.

Why not do it like everyone and buy from winols reseller database for cheap money from countries where they have to drive 120km/h max.
If the A6 3L Competition Diesel dies because of 30min++ 200km/h++ and too much of egt then just blame the customer and you are good to go.
If someone wants his egr, adblue or dpf deactivated just use some cracked one-click tool from the internet and kill all the dtcs because 90% of the "tuners" to it like this.



In reality its hard to find a trustworthy businessparner to send you master files if you dont have the knowledge to rate his work.

It starts by where are you from and which fileprice is acceptable in your 1st or 3rd world country.
Do you have a dayjob and can only flash, drive and log in your evening ours.
Are your evening ours your tuners office ours because of timezones.

just ask in a facebook group and ignore everyone who is answerking because "Wherever there is a trough, the pigs gather there"