£2500 installation, I would ask them to tell you which boat they thought you got off of? I would go back to them with a plan from above, tell them what you expect them to do and renegotiate how much they are really going to charge for it.
The way I read it is you need to spend the cash before the new budget? I really would take the opportunity to buy bigger and DIY in your own time! As edutech4schools said im sure everyone here will help out if needed. Better satisfaction in the long run and will give you a better insight into the workings of ms server and you will be able to do it standing on your head the next time ;)