In answer to your questions:
1. In order to transfer closest to the date you require, we always advise you to place the order 10 working days prior.
2. Can the base IP of the Fritz!Box 7530 be changed from 192.168.178.xxx to 192.168.1.xxx? This would greatly simplify using it with the existent network? - Yes this can be done.
3.With Zen EveryRoom, where you provide the Fritz Repeater 3000, what sort of square meterage do the combined units cover? - Up to 140sqm.
4. Can the DNS servers in the Fritz!Box 7530 be changed to third party servers or do we have to use Zen servers? - Yes, this can be changed.
5.If we went with the Zen Everyroom option can the Mesh network be configured with respect to what SSID is used? Again hoping here to replicate what I use now. - Network change: set custom DNS there too and then just change the Wi-Fi to what you want before setting the mesh up.
6.How far in advance of the migration is the Fritz!Box equipment sent out? I’m hoping to pre-configure the device to minimise the downtime ahead of the migration. - 4 working days prior to activation.
7.Similar to the above ref Zen logon details, how far in advance are they sent? - 3 working days from point of order via email.