looking for a network architecture

hello all

what kind of architecture and cable would you suggest for the given requirements.
We have 800 computers in headquarter's building and 80 computers in branch offices situated all accross the country.

We have to install the network and assign pool of 1000 ip addresses. the network must that ensures security .
We also we have to provide a mail system and internet access on all the computers. What kind of cables and routers/switches do you suggest. I was planning to put stackable switches in HQs. what architechture do you suggest.

Thanks a ton in advance for your time and response.