OK, having done lots of research around the best multizone smart heating system to go for, I'm making the plunge for Evohome.

Current system: Worcester Boiler, unvented DHW tank, 3 x CH zones (don't ask, crazy implementation due to an extension we had).
18 x rads, ideally I want 14 Evohome zones but 12 will suffice to make life easy (I'll combine the kids' bedrooms). All rads except 1 per zone have TRVs. Heating and DHW was originally controlled using a ChannelPlus H47XL with 'dumb' thermostats in three areas of the house. The H47XL is still there, all zones constantly 'on', but the thermostats were replaced by (slightly) intelligent ones which control scheduling etc.

Will moving to Evohome pose a challenge for me because I have three zones? I want these all combinined to one big 'master' zone and then 12 'virtual' (Evohome) zones. Can I just wire all three zones together (so they're either all on or all off)? Assuming I can, I think I need the following shopping list:

1 x ATP921R3100 - starter kit (controller, plus BDR91)
18 x HR924UK
1 x ATF500DHW

Is that everything?
Will the BDR91 sit in place of the H47XL?
Can I just remove the three thermostats and blank them off? (i.e. they're now surely totally redundant)
For the rads which currently have no TRVs, with Evohome am I ok to put on the HR924UKs so EVERY rad in the house is smart?
If I wanted to have 14 zones, would I 'just' need another controller?

Thanks in advance, obviously 18 x TRVs hikes the price up but I want to go the whole hog (don't understand the point of not doing it properly) - I just want to make sure I've left no stone uncovered before proceeding. I'll probably get my friendly Spark to fit it but I think it'll be the first Evohome he's done.

Gavin