For the racks themselves, we managed to get a high end server rack for free. The only thing that's bad about it, is that its kind of pain to get the side panels on and off.
I'm not too familiar with rack mounting audio gear and whether they used the front and back rail. When you buy a rack (I bet you can find some second had ones) get one that has forward and backward facing rails in the enclosure. I believe hole spacing, size and distance between rails is standard. There are a few different types, there are round hole, and square hole. I've been told the square hole are a lot easier to work with, but expect them to cost more, and everything that has to do with them to cost more. We use the round hole, and its fine because we don't move things around.
There is also different widths, the most common is the 19 inch, which is what you use for audio network and pretty much everything now days. I don't know where they measure from, but the rails are roughly 17.5 inch apart. There is also a wider 23 inch rack, but those aren't used much anymore.
If you have any specific questions, I'll do my best to answer them.