Fair question regarding the Swap ideas.
Luxembourg is not like the UK when it comes to the business of MOT..
Anything out of the ordinary here and your car is permanently grounded, unless you revert back to the approved setup. In any kind of a swap I would need to get a TUeV, which is a German certification of a job well done. That costs huge amounts of money.
I did think about the S62B50 Swap, and in fact the S85B50 (V10) swap also, at the time I had an E60M5, but it sounded to expensive at the time.
The Turbo route sounded less expensive, but has in the end cost me more. I also found out later that I had to get TUeV for the Turbo system, which I did at a price..