I don't think 2. c4 is a requirement for the benoni, however most of the lines include it and there will probably be a different middlegame that ensues if you play (for example) 1. d4 c5.
Benoni question
You could play 1. d4 c5, the old Benoni. Someone at my club plays it with great success. However, in any of the benoni's, you must play precisely and a slip could lead to a quick loss.
If he doesn't play c4 your opening will be different from all others you mention KI, Nimzo etc. The idea of a Benoni-type position without c4 is that Black will have an easier time getting in ...b5. The pawn structure would be c5,d6,g6 and eventually b5 and/or a6--Black is fine in this type of set-up
There's a possibility, if white avoids to play c4 after d4, with for instance 1.d4 Nf6 2.Nf3 c5 3.d5, to play b5 with play on the queenside.
If you like an unbalanced, and usually tactical game, then the modern benoni is a good choice for you. I am not sure about other benonis like the czech benoni, but the modern is what I play and my results are pretty good, especially in quick games because I usually know the lines better than my opponents. If white does not play 2.c4 then it will probably either transpose to the benoni later, or he will play some other d4 line like the tromp or london system or some other boring stuff.
My problem with the benoni is white can avoid playing 3.d5 and instead play 3.Nf3, which leads to a sort of symetrical english, or even worse, 3.e3 which tends to be boring and sort of cajoles (sp?) black into a tarrasch defense. There are ways to keep the game a little more imbalanced though, like perhaps putting the bishop on g7 anyway.
I've been looking into changing my repertoire against 1 d4 from the KID. I've looked at the dutch and decided it's not for me and I think I might try to Nimzo Indian.
However I wanted a second system and was really drawn to the benoni systems. The main problem as far as I can see is that if White chooses to not play c4 after d4 then reaching it will be impossible. Is this the case or have I missed something?