While you can get a block heater put on a 6 cyl engine, none of the 8 cyl BMW engines have block heaters. I looked into it when I first got my E53 in 2002. Apparently the V8's don't even have the plug in the engine where the block heater is usually installed.

I live in Canada too, and I have never had a problem starting. I have three Bimmers but only a two car garage - so last year the X5 stayed outside even when it went down to -20C.

The V8's have an alternator with higher amps and it certainly helps to kick start the engine. They recommend driving slowly to warm-up the engine rather than letting it idle. I'm surprised your X won't start when it's only 0 degrees out ...gotta' be some other problem...