There are some solutions for your network diagram about the infrastructure and security as well as capable conectivities.
The lowest cost of investment and meet you requirement is to:
1. At the branch office you should subcribe ADSL connecting HQ through VPN deployment over internet.
2. Configure Router at branch offices for your staff accessing Internet locally via ADSL instead of going through HQ gateway to reduce traffice.
3. Set up Firewall for both HQ and branch offices to allow any incoming and outgoing services for security reason. Normally we just permit SMTP/POP3/HTTP/HTTPS and VPN services.
4. Purchase MailSweeper and/or equivalnet to scan all incoming/outgoing messages to prevent from spawm and/or dirty messages before pointing to your Mail Server.
5. Install the RAS at HQ for your saleman & management staff dialing via PSTN.
6. At the HQ you should install separately all servers such File server, Mail Server, Database server, Application Server and the server for Fiwall/VPN/Gateway/MailSweeper or equivalnet.
PLease contact me at firstname.lastname@example.org for any queries/details. I will short out to help you if any.