No one WAN interface would be fine. You do not need a want interface pointing to the second office just a switch port.
If you want VLAN's that will require a router that supports them. You can go with something like and RV082 or RV016. These routers support Vlans and have more than one WAN interface if you add a second wan in the future at the main office.
You will also need switches that support VLAN's at both locations and trunking. It might be worth it to go with Cisco devices if you have the money. You can get a good Cisco router on eBay for $200 - $400 and a 24 port POE (VLAN) switch for 300-500. It's not a huge expense but it is used gear.