Consider all options
You pose a really interesting question, and very first question I would ask is... What level of availability and support do your charity require? Having worked in the not for profit sector a few times I've come to the realisation that actually maintaining availability (and subsequently operations is really important for cash strapped orgs) - one big fat server sounds great but it's all of your eggs in one big basket, two servers is better if the business can handle a 50% loss of service for a sustained period - otherwise I'd be looking at 3 or more to minimise the chances of loss. With that in mind you would probably want some level of shared, and critically resilient, storage AND backup - if your charity is anything like the places I've worked with previously then having critical data actually held on the desktop isn't unusual. An open source product such as OpenFiler running on a storage heavy server would provide a useful repository for virtual file-systems/disk images and wouldn't break the bank.
Ahead of all of that however there are probably a few things I would personally consider:
- What are the applications that your customers are running? Do they have specific applications that require Microsoft Windows on the desktop? Are they likely to respond well to running in a virtualised environment - i.e. are there any applications that require specific hardware (dongles etc)?
- If you are considering virtualisation for the desktop (even just 30 of them) then it's important to understand your performance requirements. I'm guessing, given the implied age of the desktop, that there aren't any "heavy" applications in use such as CAD tools - that said most charities these days make use of higher end graphics and marketing packages for event promotion - so again worth take a good hard look at your application landscape.
- Does the charity have any existing virtualisation for its servers? If so then it's worth considering if you have a pre-existing reference solution to build on so that you're not building up any divergence in supportability. If not then it's worth considering what benefits you could realise from having everything virtualised - just a thought.
- Are they absolutely married to Microsoft Windows as a desktop operating system? Regardless of whether you virtualise or maintain a more traditional desktop estate considering FOSS alternatives to both the core OS and the productivity applications is an absolute must. Both Ubuntu and LINUX Mint offer excellent desktop environments for end users - and to be honest, the growth and maturity of most business type applications for desktop LINUX is in a great place right now. If you're happy to support a LINUX environment then having an Ubuntu based environment gives you low(er) hardware requirements than Windows AND the server product offers virtualisation capabilities as well which could give you an easier path to VDI down the road.
- Have you checked whether you actually need to have ANYTHING on site (aside from a terminal to consume a desktop from). If you're considering the spend on new hardware, licences, backup and support then it's well worth doing the math to see if a hosted desktop with either a dedicated provider or creating a home baked solution in the Cloud might be worth a look. Is it even possible that your business uses so few "traditional" line of business applications that they could consume cloud based services instead. Most vendors of IaaS and SaaS provide good levels of discounts for charities - you may find that actually spending the effort to identify and integrate a series of 3rd party cloud solutions gives you better mileage, less of a management headache and (potentially) better availability than running your own solution.
I realise I've not really provided you with a "do this or that" type response but I hope that maybe some of those insights might held solidify which choices you make. Either way, good luck with your potential project.
Cheers,
Phil.