You mentioned that the car was sitting. In addition to spark, the next culprit could be the fuel pump. The pump may fail (the fuse is fine so it would appear that the pump is fine too but it could be pooched or plugged with debris). If you have a jack and jack stands. It's easy to get at and they are cheap to replace. Be sure to relieve the pressure on the line before you disconnect it and always remove the Neg cable from the battery before you begin.

https://www.fcpeuro.com/products/mer...p-0004707894-1

If not, contact a local repair shop that specializes in MB and BMW. Every city has a few. Go to the shop, explain the problem and when it happens. They may be able to point younin thebright direction or correct it for you for an hours labour plus part.

Please keep us updated. I would like to know the cause once you've isolated it.