I will talk only about the file-sharing since I'm not familiar with all the apps you mentioned.
There is no need to run OS X Server to do file sharing. Even just the client version of OS X does file sharing fine. Check it out: it's in the 'Sharing' icon in System Preferences. The only difference is that it's a little slower, and (I think they still do this) it's limited to 10 users at a time. Since OS X Server is just a cheap addon to the Client, there's no need to actually buy a new computer with Server on it. You can buy the client, use it as a client, then pay a little money to upgrade to OS X Server later when and if you want to.
The computers don't need to be running the same version of the OS. A 10.6 client can talk to a 10.8 server and vice versa. They will happily talk over WiFi, ethernet, carrier pigeon, tin cans with string, or any other way of connecting to the internet you can find. If your home WiFi router can have two computers connected at once, it'll work.
However, you mention one thing that worries me: iMovie. Movies contain a great deal of data. Editing movie files involves a great deal of moving data from storage to the computer and back again. If you try to do movie editing using a server as file storage, it's going to be extremely slow. If you do it over Wifi it will be too slow to be any fun at all.
For the other apps, where you're only playing with a meg or so of data at a time, any of these networking setups will be fine. For iMovie it will be better if you keep your movies on your hard disk while you're doing your editing, and perhaps just move the whole project back to the server when you switch to something else.
Hi Simon, thank you for your reply.
That's really helpful to know. I vaguely knew in the back of my head about file sharing but have never had two machines I've wanted to do that with before so it never occurred to me I could already do it!
That sounds like good advice on the movies. I want to move them all off my MBP & onto the new machine (when I eventually get it) and work with them all on there instead of the MBP. (From the reading I've now done I think if I use iMovie 11 on the new machine I won't be able to open those in the old version of i Movie)
Thank you :-)