What network architecture would you choose for files and mail?

Hello!

I work at a company that plan to join an other company. During this plans there have come up some questions among the network between the companies.

To day booth companies use separate LAN (Windows) and also use separate mail-servers.

Together all employees would be around 500 I guess.

The companies is located at two differnt locations and wants to remain like that.

Employees some times need to travel between these two locations. When they work at one office they would like to be able to have access to the same LAN and files like they have access to at the other office. That means we need some filesharing-service I guess.

The employees also would like to be able to read the same emails so my guess is that all emails must be stored at one mail-server never in the client.

I also guess that if this is possble they for sure would like some encryption during the transfer of mails and files. My only guess is to use IPsec, is there any other solution?

All this ofcourse demands that all users save there work at the fileserver.

The main issues is to be able to read/write mails and files over the network.

So if you could advise me in some way how an reliable and scalable network architecture for this would look like please let me know.

Best regards
Fredrik

BTW if it matters my guess is that booth companies would like to use Windows in the future as well.