HIK intercom should be able to do this having a play with one tomorrow
Comelt will work with ease but will require dongle for each PC it is running so cost may be a problem
Three of us heading over to see a couple manufacturers and get a catch up with a few guys who do work for us in England and Wales. It is very expensive and would prefer Birmingham.
I would be very wary of putting 256 cameras on one NVR what is the throughput on one of these. We keep on coming up against people who recommend putting 50+ 3 & 4mp cameras on 64ch NVRS which have 150mb total throughput which just does not work.
For our larger systems we use Avigilon and build our own servers never has any issues.