If you have had the vehicle from new, then it went seven years before freezing up. It is your call, but I wouldn't worry very much about the cold weather kit. If you are doing the separator, it is just as easy to do the cold weather (insulated) parts, but it isn't critical. If you do a lot of short trips and the engine doesn't get warmed up properly, you should plan on cleaning out the hoses every couple of years, as a preventative maintenance step.

I would have an independent shop do it given the price the dealer is quoting. Your buddy is right that the separator (which is like a PCV valve) is not hard to replace. It is the number of hoses, and the tight working space, that makes it a challenge. Not technically difficult, just a PITA.

Recommend you stay with the oil that BMW recommends. A thinner oil (0-40) is no less likely to build up condensation from short trips, and that is the root cause of the failure.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue