We were thinking to move to VOIP tech for sometime now
Good, few pointers (you know the most likely).
1. VoIP does not support emergency number dialing
2. Make sure you network is smooth, eg introduce QoS etc.
3. Security overlays can introduce call deformation (not much but they can).
4. WiFi although great technology it is not really a "healthy" technology (it cooks water, that is why it does not like rain and fog).
We have about 20-30 extensions and some warless phones as well. We also have 5 fax machines and voicemail.
30 Extension, you looking at the Professional Edition. I assume your Wireless phones are DECT so and are not included in the extensions. I would go for the enterprise version these are the reasons why:
1. Each teacher their own extension
2. Each teacher does not need a phone but might want to have voice mail they can retrieve voice mail from any phone by dialing their mail box or even have the voicemail forwarded to thier email box.
We are also establishing another campus. We are looking at reducing our cost and linking both campuses.
Love it when they do that, reducing the hardware cost by increasing the recurring costs

.
My advice is (depending on the infrastructure) one PBX per campus. This introduces redundancy.
Just wondering if 3CX is what we really need to do the job. Would the free version be good enough for us?
If not how much does the commercial package cost?
No you you do not really need 3CX, you could install a mega bucks type PBX, but why would you. 3CX can easy do the job running on a Windows machine in the corner. For sure you have one somewhere.
Does the commercial package require a dedicated server?
Nope, the application is very sociable and can run on any windows server doing some other stuff, it can even run on a virtual machine I believe. For example if you have a Domain Server you can run 3CX on that. But again it depends how much load you want to put on 3cX and how much spare capacity you have.
Here is a link to the different versions.
http://www.3cx.com/phone-system/enterprise-features.html
Ok now the disclaimer:
I am NOT a 3CX employee, neither do I get paid by them for services. I am a 3cX user (and a happy one that is) So my comments should be taken like that. Re prices and what version, I am not in the position to quote other than what is on their webpage.
Hope this helps
Henk.