Unless you actually plan to use the fancy features of the Defi controller/gauges, I don't think they are worth the price. Good old Autometer and similar gauges are significantly cheaper, meet most folks needs and are available at Summit.
I can't really add anything to Scott's list other than to say once you get past oil/water temp/pressure, Boost, EGT and A/F all go together. Boost by itself is just a cosmetic/show off gauge. Once you need one, you need the other two IMHO. If you want a boost gauge to make sure you aren't over boosting, you better be worried about EGTs and AFR.
FP is really a troubleshooting gauge more than an operation one unless you are racing without a fuel gauge.
