Most MSSQL Inexpensive Licensing Option for Web-based App

Forgive my greenness on the above.. we are trying to understand what it will take to migrate functionality from an existing LAMP based application to something that uses MSSQL on Windows Server...

We are self-hosting an application that will draw it's data from multiple unique customers who will be log shipping to our centralized server. Each customer has a dedicated always on IPSEC connection, and our main box will be receiving one-way replication to our server. There will be a dedicated VMware machine that will be the host of the MSSQL server that is running 2008 Server R2 (server is not on a domain). Each customer's users will login to our webapp via the public internet (currently planning on running IIS on the same box), and the app will connect the MSSQL databases specific to their account.

This is a niche app, so there would not likely be more than 4 or 5 unique customers on this instance, each with perhaps 5-10 web users.

In addition to the above, we would need to administer the databases via SQL Management studio, and there could be 2-3 users connected at the same time to the server.

Am trying to do this as cost effectively as possible... Is something like this an option?

http://www.ebay.com/itm/SQL-Server-2014-Standard-with-10-CALs-2-Core-CPU-License-64-Bit-Software-/141838962264?hash=item210642ce58:g:xogAAOSwWKtUrlRA

or is the web edition and option? I'm looking to be creative.

std edition would do it