BMW winter wheels offset
 

Does anybody know what offset are the official BMW 19" rims? I ask because I feel that one could say quite a lot of money here, however I couldn't fins the exact sizes of the rims. Any help much appreciated.

You don't mention a specific style wheel, but a cursory glance at the wheel site, pasted below, shows that most 19"
X5 wheels appear to be in the 45 front, 48 rear offset category.

Good wheel site; it's in German, but just use the drop down tabs and plug in
E53 or E 70, or...
Felgen-Katalog - BMW-Treff - der Treffpunkt für alle BMW-Fans und -Fahrer

GL,mD

I believe the biggest potential problem with purchasing a 19" winter wheel set up is not so much the offset per se - the problem is the inner part of the barrel, which needs to clear the brake calipers at the front.
Remember, the front brakes are HUGE. This is why BMW has a dedicated set up for the XMs.
You cannot simply purchase a set of 19" rims with the correct offset - chances are, they will not clear the brakes.
I have not seen ANY other 19" wheels that fit the bill, apart from the OEM 298M set up.
I also toyed with the possibility of ordering 20" wheels, and putting winter rubber on them, but in the end, purchased the OEM set up - it was less hassle, and they work incredibly well.