Hi and welcome to the forum!
You are doing exactly the right thing when it comes to sizing for your network - both looking at the current bandwidth availability and considering the future too. This page is really useful for this of course as its a full Balance router side by side comparison. http://www.peplink.com/products/balance/model-comparison/
When I am sizing a Balance for a deployment I am normally looking at 4 key things:
>The number of WAN links (ie number of WAN ports on the Balance)
>The available WAN bandwidth across those links (and so router throughput)
>Whether SpeedFusion VPN will be used (and so the number of supported Peers required)
>The type of traffic that is likely to go over SpeedFusion VPN (if its in use - so the SpeedFusion VPN throughput).
The number of recommended users is a ‘soft’ guideline that just acts as an indicator of suitability - that said, as the number of users increase on a network then the number of active IP sessions will also increase, and each of these sessions has a tiny CPU/Memory overhead. So long as you’re not planning to have all 80 users each running 100’s of their own client sessions then the Balance One should cope fine (I know we have tested the Balance One to the 5000 concurrent session level which worked great).
Its worth talking about the firmware feature differences between the Balance one and its ‘enterprise big brothers’ too whilst we’re at it. Obviously the B380+ all support SpeedFusion VPN which is a big draw for our multi-site customers, and this does add to the price as its a premium enterprise feature. The rack mount form factor of the enterprise boxes are also very different of course, with its front mounted LCD allowing for browserless reconfiguration / visual status indicators and admin password resets too.
Other enterprise features found on the 305+ include:
>Hardware High availability support (VRRP)
>Drop in mode
>Authoritative DNS support
>Inbounnd Load Balancing
>Higher PPTP user count (with Radius Authentication)
>Larger number of VLAN support on the LAN
>More advanced AP Controller Functionality (the Balance One only supports a single basic AP profile)
>QoS User Groups (with bandwidth reservation)
Hope that helps clarify the differences between the Balance One and the Enterprise family. I personally have not used the Balance One for a user count as you have described, but maybe someone else here has and can share their experiences of that?