I think I would prefer an unheated garage as heat can accelerate rusting I believe. I may be wrong on this, so maybe someone can clarify.
Mine will be 3 months in an unheated, but insulated, garage and on the battery trickle charger.
2012 135i E82 MSport 6MT Jet Black
Previous BMWs - 2012 135i MSport LMB DCT(sold), 2011 X3 35i MSport VR (sold), 2009 M3 Sedan AW (sold), 2008 535xi MSport AW (sold), 2007 328xiT Barbera (sold), plus 11 others in the 70s & 80s.