I would look for an 06 if you can swing it. That model had a motor upgrade that seems to have less issues than the 645 (i.e. Coolant pipe). I've had my '07 for about a year a half with no issues. Either way you (and she) are in for a treat.
'07 650i Coupe - Black Sapphire Metallic
M6 bumpers, skirts & wheels
BMW M Cross Spoke Composite 101 Rims 20"
HUD, Comfort Access