Thanks for all your advice so far. There is definitely a lot of food for thought. In response to some questions (and to fill out a few gaps in information)...
We have 1/2 day support every 2 weeks from the county ICT provider for what we have already. The server is a similar age to the desktops (2006) - running windows server 2003 (I think). Our MIS is county standard too which we also subscribe to for extended support. The school layout is 6 rooms basically along a long lateral corridor from one end to the other in a single storey. We have exceptionally poor rural broadband speeds (don't we all!)
I like the principle of maybe keeping the old desktops and supplementing them with newer portable machines. I worry about spending resources adding more hard wiring into an old concrete 1950s building. All in all it's a real tough call.... You really want to do right not just for the kids at school now but to try and make sure the next intake also have exciting, relevant curriculum experiences too!
Thanks again for all your thoughts and keep them coming (if you can!)