T1 line with voice channels
http://www.newegg.com/Product/Product.aspx?Item=N82E16833124190 with DD-WRT or OpenWRT using a PPTP vpn solution? Or… just use freeNX thinclients/server?!?
Astrisk system
Thinclients using Terminal Server at branches. Laptops at main office for executive staff with Vista / Microsoft Office.
Linux Terminal Server Project on Gentoo with KDE4, KPhone, Evolution/Thunderbird…
FreeNX and LTSP?
Postfix with MailScanner antivirus/spam. Can run on Firewall machine.
Gentoo iptables firewall
Using my vpn solution already documented here. Can be run on Firewall machine
Pure 1GB network for main office. Need a 16 port cheapo linksys or something switch.
Cable/DSL accounts. With max upload speed possible for each location.
Estimates of 40kbps for voice and 50kbps for terminal. With 30 vpn users you'll need 3Mbps up/down at least. I'd go with a 5mbps up/down if possible.
webERP - Free OpenPro - 30k