i would suggest egt and oil, get a pretty good idea how things are going from those two. especially if you like to run in boost
a wideband afr is nice if you are going to be doing any of your own tuning, if you are running an AP then i am not certain it is as necessary. there are other things, like a nice matching functional clock, if you are running any accessories like thumping stereo, a basic juice meter. the list goes on, but i think the two for certains are egt and oil. silly question, do you already have a boost gauge?