Can I just setup a clean install and image this using Windows 7 backup and then restore this image to all the other machines (changing computer names and rejoining to active directory of course)?
Please, good sir, please. Until you get WDS, or WSUS, or SCCM up and running (all tools to help you automate the distribution of images) just follow the steps in this guide. The guide will walk you through, step-by-step, how you can create your first image using WAIK and Sysprep.
It will save you hours of headache, I assure you.
PS - Don't install anti-virus software while making the image, as it has a tendency to cause a problem. Everything else can be installed without issue.
Why would you do it this way? It seems like a lot of work.
Yes you can probably do it. There are some problems because of the lack of being able to sysprep the machine before running Windows Backup. You'll run into naming conflicts when you first turn the computers on. They'll also have duplicate SIDs (not so important for most things but it does effect things like WSUS reporting).
A better option would be to take some time to learn Windows Deployment Services. This will do what you want it to but make computer naming, SID regeneration, and deployment much simpler and quicker. An added benefit is that it's included with your Windows Server licenses.