Don't need to worry about that
Yep deffo need to enjoy this process as once its done your like what else can I get on with lol
You can do the M57 which I think is out of the 5 series, ive never personally used that box but know it bolts to the PMC adapter plates. The one I had specifically was the Getrag 420G box which is from the E46 M3 and reasonable value at £600 - £800 for a good used Box.
The only issue with them is they are a little noisy and don't have a built in speed sensor, however you will notice on that car there is a black box on the transmission tunnel which is a decota box and used to convert a ABS sensor speed pickup into the speed for the clock in the dash.