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1. Okay, let’s try to break it down :
12 users that need to always connect remotely ==> You need to reserve upload bandwidth for them and preferably research the term QoS (Quality of Service); it’s an option in your network setup/router that can allow you to prioritize packets and reserve bandwidth for some defined services. I believe that will be the VPN service, coming from experience, iI usually remote connect machine via VNC over SSL (if I had the chance/UI on the remote machine), so the bare minimum for that connection is 50-100 KB/s mutual speeds from the server side (upload), and client side (download). Keep in mind that VPN speeds are less than VNC, and its relies on the stack that your using.
A lot of folks like OpenVPN but state there is a lot of other commercial offering that is much faster, in the same environment, but all in all, I think ensuring at least 50KB/s per user from those 12 is good number, thus 12*50 KB/s = 600 KB/s.
You also specified that those dozen will mostly work on small files from what I have read, so that is good number, make sure to try to prioritize them via QoS, because that’s the reason it was created.
On another note, usually you set up VPN to far-access clients, thus they may connect at different times, so that will help reduce the load on the bandwidth, and not simultaneously.
Other than that, most packets IMHO will run through LAN, ensure you have proper software if you’re the I.T guy there, to maximize LAN usage (easy file sharing/NAS, Chat/XMPP server with file send support/pidgin,.etc) and by that I mean try to make the LAN the preferred way for your personnel to exchange files, since they usually rely on 3rd party products for its ease of use, especially in sharing, so for those 38 left, depending on the work load and type you can calculate things better. For example if you working in 3D design company, then you will be dealing with large files, and having fast upload is a must, but I have feeling that your office is dealing with small files, so my guess is 3-4 MB/s for best price/performance.
You have already taken the first steps in evaluating your requirements by looking at the number of employees and the types of applications in use. Also, important is the types, and therefore sizes, of files that will be emailed, stored in the cloud, etc. To really get an accurate figure, you are really going to have to monitor bandwidth usage to get a picture of current usage, but that isn’t really an option for you at this point. This also puts an emphasis on scalability of capacity for you, in case your initial estimate is too low.
There are a number of free bandwidth calculators online that will give you an educated guess, but to really nail it down, you are going to have to monitor it and see what real-time use actually is. Still, it doesn’t hurt to give one a try and see if it is in the same ballpark as your own bandwidth estimates. http://bandwidthpool.com/bandwidth-calculator/