WAN Load balancing - what am i looking for ?

Looking to do load-balance between WAN's for a companies main office. What type of hardware am i looking for ?

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some "specs":

• "Head Quarters" has
  • 5 WAN's (same ISP) at 35 Mbit/s down / 5 Mbit/s up (speed-test) (V-DSL)
    • expected to grow to 100/200/400/1000 Mbit (soon^TM)
    • each WAN supplies IP-Telephoy (including a SIP-Trunk)
  • 20 work stations (back office)
    • expected to grow to 40 within the year
  • 10-ish Wireless-clients
    • expected to grow to 100+ users with 2-3 devices each as 'the company moves away from "pen & paper" and embraces the 21st century' (quote) and allows BYOD.
  • 1x IPsec tunnel for a branch office (currently 5 Mbit/s)
    • 5 work stations
    • a plethora of Wireless clients
    • massive increase in bandwidth planned (due to remote backups (HQ <-> Branch office) being considered)
    • additional branch office to be integrated at some point (new tunnel - same size as above) - contingent on their connectivity increasing.

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Backstory:

• HQ and Branch Office have different 'contractors' responsible for their respective connectivity/network.
• Second (planned) branch office has "in-house IT" and runs a paper-less office (for years) - looking forward to do remote-backups into "HQ"

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The Problem:

I am being "quoted" Vendors (Watchguard, Sophos) and prices (ballpark 8.5k usd) , but no one is getting specific on what type of hardware this is going to be.

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Addendum:

1. (for the lack of proper terminology) I am looking for the type of "load balance" that mode "balance-tcp" on LACP with openvSwitch does on my proxmox-servers) - yes, i am a sysadmin/'server-admin' in charge of herding cats (here we are again ...)
2. 35 Mbit/s is the max a single connection can provide (the line is supposed to do 50 Mbit/s)
3. the tunnel between HQ and branch-office is at current fast enough. It is the WWW-usage at Main office that is crawling (1 connection utilized)