Addressing a few of your concerns:
Mike A.'s ZRC is the old JBS race boat rebuilt by Nor-Tech better than new and is one beautiful piece!
The difference between the newer 4 step ZRC's and the 2 step models is very minimal! The 700 NXT 4 steps have seen 115-118. As you can see below, my two step with 700 Potter's runs right there with them! I have been in both bottoms and there is no real world difference in speed or handling.
As for saltwater, if the boat is well cared for, flushed out and stored properly, there is no problem or detriment to buying one. My boat (2003) has spent its whole life as a saltwater boat and I just got a complement this weekend from a guy who thought it was brand new.