Tip: I wanted to buy Avata 2, but I found that goggles 3 are not compatible with my mini 3 pro.

Now I'm undecided, I buy Avata 2 for 1200 euros and patience. Or do I buy used integra + controller v2 for 450euro and use it with the mini 3 pro? Or Avata 1used with goggles 2 an batterys extra for 850?

Advice?

Id probably go the used integra route as there is no guarantee the Goggles 3 will get Mini 3 support.  

Those are completely different approaches - Avata 2 is a Great drone and will give You a completely different perspective allowing You to fly low and in confined spaces where the Mini is not good for

I would go with the Avata 2.  While flying Avata 2 in FPV mode, there's no need to use your FPV equipment with the Mini 3 Pro.