A closet fan of them too !
I had contemplated one a couple of months back but the back seats were just too tight (legroom wise) for a 2yo & 4yo. There's zero legroom behind the driver (if you're over 6ft tall) and it would have meant putting the 4yo in the front seat, but potentially too close to the airbag for my liking. I reluctantly ended up in a non BMW saloon thereafter !
The running costs of the 645i are just far too high (even including man-maths because it's a V8). The 635d are a lovely machine alright but there's a big premium to pay for them over the petrols - you'd need to be doing starship annual mileage to make that worthwhile IMO.
Depending on budget, but you could go for a 2008 on LCi 630i which had a c. 13bhp power increase to 268bhp (an 0-60 of 6.4 seconds is pretty rapid in anyones book) with the added benefit of €750 motor tax. The LCI petrols seem pretty thin on the ground though....