Yes, 56 plates will generally have the BWA engine code and the earlier ones will have the AXX engine code. The BWA is preferred and is regarded as the better engine.
Personally, I would read up on the optional extras that you can get with this car - in my original searches, I didn't know the half of the them - luckily I got a high-spec Apr 2007 one with 97k for £6.2k. It's now on 108k and it hasn't missed a beat with correct servicing.
I got a high-miler as I didn't intend to sell up until 3 or 4 years down the line anyway, so didn't think the retained value would be that high in a 100k car at 10 years old, so just bought cheaper then instead.