You can find the bluetooth retrofit instructions on here - in the DIY section I think - it looks like a right faff and you may find it impossible to remove the bits of the loom that you need (the retrofit kit maybe different to factory in this respect - i.e. the factory loom for a BT car is basically different). I gave up on buying a particular 335d Touring without BT as a result - I did look into it as the car was otherwise perfect.
It seems that they don't bother wiring non-BT cars for BT so it is not that easy.
I would just make the effort to get a car with the right spec, and try and waggle your blacklines back without having to buy the car & all the hassle (give them £20 or something). Just forget the rest IMO.
EDIT - one option maybe to buy it and offload the car straight to a specialist BMW breaker like FAB direct - and see if they'll give you the blacklines and alpine kit for your trouble.