Law / Vehicle export from Poland [3]

Hi everyone.

Me and my friend are planning to export some BMWs from Poland, due to how much cheaper they are here than they are in Slovenia.

Cars would be used for racing/drifting or as donor cars for our "legal" cars.
I already found a thread on this forum stating that a buyer-seller contract and a proof of insurance is enough to safely travel from Poland to Slovenia, but the post was from 2012, and I'm sure things have changed since then.

Can anyone give me any pointers what do I need, what to watch out for ? I also read that there's a tax for anything valued above 900PLN?

Thanks in advance,

Žiga