Having done this very thing over the summer holiday I would say this: it all depends on how much time you've got in your school to dedicate to troubleshooting network issues.
We work with a company called Softegg who provide technical support, and they setup the servers to allow for a Vanilla network, but I had to do the workstation/user side of things, and it took the entire summer holiday to sort out (creating images that worked, setting up WDS, setting up KMS, transferring old users, creating new users, creating a working mandatory profile, adjusting GPOs to work with Windows 7, setting up Printers etc) and I'm still ironing out the kinks in the system.
You can definately emulate, or indeed improve upon, an RM system and spend less money doing so. For instance, App Deploy does exactly the same as the RM method of creating applications to roll out across the network. However, the time it takes to setup the network, and the knowledge you might need to gain, has to be a large consideration imo.