be agile - just grow it and and don't be afraid to change as you go a long. Pick something very simple to start with and see how it goes.
Here is my simple environment. Decide what you want to do on a desktop to implement the joel test question #2. This in my experience is pivotal. Once you can do the complete build and test on a desktop with one (or a couple of) clicks then you have the basis for an automated build machine.
The trick is get this going quickly using what ever tools are at hand quickly - e.g. VSS and Cruise control, MSBuild etc, and tune. You should be able to get a pretty good (80/20 rule) in a few days of iterative effort.
Edit: Just to answer comment.
Yes you are correct is is be very beneficial to learn from other people experiences (not just their practises). However having set up build environments in may different companies I can say the best thing is once you start with some else's complexity you end up fighting other peoples requirements and assumptions. Yes I've now got experience and I can use that as a metric to start easily.