1: Exposure suit, be it wet suit or dry suit
2: B/C
3: Regulators
Here is my reasoning for my priority of gear selection, forgive me if I ramble.
I think a relatively new diver should be comfortable when learning to perfect the skills of diving.
Removing the unwanted stimulus of being cold will help a diver concentrate, more studiously, upon the task at hand, learning new skills. A proper fitting exposure garment will enhance this ability. At the same time, the diver will learn the propreitery buoyancy characteristics of his/her chosen exposure suit.
As we all know, buoyancy is one of the most, if not THE most, requisite skills to be mastered in order to become a proficient diver. Becoming accustomed to the particular buoyancy characteristics of the exposure garment without moving from the differing buoyancy characteristics of one rental garment to another will help the new diver achieve this plateau more effectively.
Therefore, I recommend buying the exposure suit first, and the B/C second, for the same reasons. The B/C will have some effect upon the buoyancy, but to a lesser degree than the exposure suit.
One of the reasons I put the exposure suit first is because of certain personal hygiene concerns.
I recommend buying the B/C second because it helps the diver become quickly accustomed to the locations of clips, snaps etc. Using various rental B/C's will retard the diver's ability to automatically locate fasteners, etc.
The reason I recommend the regualtor last is because once the diver puts it in the mouth, there's not a whole lot else to worry about.
As to the concerns of the servicing of rental regulators, I imagine the last thing a dive shop wants is a law suit due to the injury of a diver due to a malfunctioning regulator.
From what I've seen with respect to rental gear, the regulators are serviced best followed by B/C's and then wet suits.
Needless to say, there are 9 differing opinions on this matter, so the choice isn't that great.
the K
