A dual-homed Internet design with a single edge router and site-to-site VPN topology best matches the stated cost and flexibility requirements. The customer needs redundant WAN links, has a limited budget, wants easy future bandwidth increases, and does not require consistent end-to-end QoS on the branch routers. Internet circuits are typically less expensive and easier to upgrade than MPLS access circuits, making them appropriate for small to medium-sized remote branches when strong QoS guarantees are not required. A single edge router also reduces hardware cost and operational complexity compared with dual edge routers.
WAN MPLS designs can provide stronger service-level guarantees and QoS treatment, but they are more expensive and less flexible for rapid bandwidth upgrades. A design with dual edge routers improves device redundancy but conflicts with the limited-budget condition. Therefore, a dual-homed Internet solution using a site-to-site VPN overlay is the most appropriate option. The architect should still validate encryption requirements, failover behavior, routing preference, and whether the Internet underlay can meet application performance expectations. Reference topics: branch WAN design, dual Internet connectivity, site-to-site VPN, cost-driven WAN selection.