Ok, so the seller confirmed me this morning he has sent the A-Post package with the full shimano triple 6603.
I should then receive it tomorrow morning, the postman comes allways at 8am to my home.
If I get the package then I'd take the morning free, go to Veloplus in Wetzikon with the bike and try to change the group on the bike.
I expect to finish before noon. Maybe litespeed would come to help.
If I am succesful I'll be set for the Thursday ride with the newly setup mountain bike ( no knobbly tires , but I could put a suspension fork on it

).
If the package arrives on Thursday I'll try to do the same on the Thursday morning but then It'd me more problematic with the work.
If I fail to do it. I would probably not have the bike ready to come on thursday.
Let's hope everything goes ok.