If your installing a mechanical oil temp gauge, you'll have to make a hole in the oil pan for the probe, unless you have an oil cooler, then you could put a manifold in and plumb the probe in there. It think fuel pressure, oil pressure, and water temp are probably the most relavent. My .02 on the latter.
Boost gauges fall in the same catagory as vaccum gauges to me. Nice to look at but not to terribly important unless there is a specific need for them (course if I had a supercharger I would probably do the same
) Know if you were looking to measure the relatove worth of mods it might come in handy since a reduction in boost tends to indicate better heads/intakes, while on the other end creating more seem to indicate a gain in airflow.