i dont know anything about renderfarms, but from a logics point, if all are diferent builds you should have some problems setting them all to render 1 frame... how i think it works is that each node will render a diferent segment of the picture, but if all have diferent computing power it should be hard to sync all together, if it is possible you will always have to wait for the slowest node to finish its part... so essentially the only thing thats gona matter is how many threads can be processed at the same time (how many slices of image)
but again i have no idea you should try what he said lol =)
I have a small render farm set up and do exactly what you're proposing.
You have two options;
1) Use Backburner. You're getting only one computer at a time because you have to set Split Scanlines (I think, I don't use the regular render engines) in the Render Settings to have multiple computers work on it at once. Make sure Use All Servers is selected in the Backburner dialog box.
2) Use vRay. vRay natively has multiple computers work on one image at a time through Distributed Rendering. It costs about $900 per license but has been well worth it.
With vRay's recent update, you can combine Backburner and their Distributed Rendering. I've had some mixed results because there seems to be a bug that saves over the last image when batch rendering... Oh well, you can't have it all.
Its really been a time saver for us. We just got four new but standard desktops to make up the nodes, and it has cut down the render times to 1/4 of what they are with a standalone.
Good luck with the interiors. I do both interiors and exteriors and I prefer the latter.
You may want to try posting on The Area or somewhere aimed at 3D graphics.