[sperry]

My personal preference for gauge importance on a WRX/STi depends on the level of work done to the car:

Stock, or relatively stock, street driven: boost > oil temp > water temp > EGT > oil pressure > AFR > fuel pressure
Modded, or track driven: boost > oil pressure > EGT > oil temp = water temp > AFR > fuel pressure

If you're short on cash and really like the Defi gauges, I'd spend the money on just the Defi boost gauge and controller now. Then add the other gauges later on. I would expect the Defi boost gauge/link controller to cost around the same as a full setup of other gauges and a pod. Just slap the boost gauge on the steering column, then you don't have to worry about losing your clock until later when you add the other gauges.