Project planing - with Linux based solutions

Hello,

Please suggest or share any of your experience for the following requests:

Request:
1.
Creation of the local network.
The company will be in the field of IT.
10-15 persons. 10 will be physically in the office.

The local network should be secured and all the files and documents should be encrypted.
The network should have possibility of VPN or some other secure distance connection to internal network (to gain access to internal documentation or software)
Type of open source firewall? Should it be on separate server?
For the router: not sure yet, will see the budget, but probably will configure some Cisco router or Linux version (any suggestion for linux server router os?)

Request:

To create network architecture, including solution of back-up server.
To choose type of servers and OS that will be used.

  1. Creation of secured email communication.
    All the emails should be encrypted.
    All the emails on the email server should be kept in encrypted format.
    Need to choose server OS.
    Need to choose server type that will correspond to request.

  2. Choosing and integrating internal management system - ERP software.
    Example: Alfresco or BitrixSoft)
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So my questions:

If for the beginning will decide to use only open source free based software.
What can be sugested.
For the beginning, for example, will have 2 servers.
1 server for: local network, files sharing, hosting of intranet website, email.
What open source virtualization software can be suggested to use on this server to separate resources between Main Local server, Email Server, File server, Intranet hosting server?
What can be suggested from Linux distributions to use as main local server (user control, access ect.)?
What can be suggested as Linux email server? (should include encryption as describe below, option of calendar, tasks and all futures as closed as possible to Exchange of Microsoft) - and what are the most user-friendly options?

Is their any suggestion for ERP software beside Alfresco or BitrixSoft (I tested Bitrix - and it is really good, but expensive for the beginning...). Is their any good open source ERP solutions?

  1. server for back up. What solution can be suggested here?

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Also, for later steps of software development need to choose the standard of software process development.
What can be suggested?

Examples:
Rational Unified Process (RUP)
MICROSOFT SOLUTIONS FRAMEWORK (MSF)

Thank you in advance