Who is going to manage the service, and the infrastructure?
I'd be looking to see if SAGE provide a SaaS (software as a service), or looking at Cloud based offerings from Amazon or Azure through a 3rd party vendor providing the support for IaaS (infrastructure as a service), and managing the service for you.
It'd be best to look at all the options before splurging the cash, and then realising that either:
- your performance is poor
- you have no support
- you dont have the right licence
- your scalability for the now 300 users isn't achievable without spending a whole load of cash (I know you said 50, but is that the realistic maximum over 3-5 years?)
My big question would be.... Why have you been tasked with providing an IT solution when you clearly don't have much IT solution experience?
Has a business bod asked you to buy a server to run his application for his workers? If that's the case, then you need someone to provide a managed service really.
You could just look at the minimum specs, licence model, and then buy a piece of kit to fit - but that's only part of the solution.
If you like, I can point you in the direction of a consultancy company I've worked with recently to provide a Cloud solution for a distributed IaaS cloud offering, which was a managed service. Let me know your email address and I'll put you in touch, they speak plain English, and are very reasonably priced and flexible.