The Benoni strucure makes sense as Black only if white's pawn is at c4 already- else that square is a great place either for the Bf1, or the Ng1. Black isn't lost or something close to that, but he certainly has an unplesant position to defend.
Old Benoni as the best anti-d4

As an d4-player. this is not really a threat.

Just speaking from experience I always feel like I get a good opening position against the Benoni... and I just know the basic ideas of the opening, I haven't even studied the lines. I think Benoni players have to try far harder than their opponents to even equalize. For the most part whites moves follow the same pattern and can be figured out OTB... I know it's unusual, but you always have to ask whether the surprise you get is actually paying off and worth compromising your position. Because at some point the game complexity is such that neither player knows the position, at that point you just want a fundamentally sound position over a surprising but compromised position.
The only real reason to play the Old Benoni, IMO, would just be to learn the lines and then move on.
I do like the benko but I would probably not choose to enter it via the Old Benoni.
Playing the old Ben Kenobi. I think c5 by itself can be good at the club level, in some blitz game in case the other player doesn’t know what they are doing. I haven’t really faced this as white, but according to memory white should play d5 and probably f4 at some point, and white should be better off.

There is so much wrong with this.
1.d4 c5?! 2.d5! Nf6 3.Nc3! Does not make ...b5 easier to play for Black. The moment Black plays ...a6, white can take the positional approach with a4, or depending on the line by Black, White can get a raging attack down the middle. It winds up either a Schmid Benoni or Half Benoni or Closed Benoni or Snake Benoni, all of which are good for White.
I would typically play a4 and take the positional approach with e4, Be2, Nf3, O-O, and then Nf3-d2-c4, assuming Black played ...d6 and not ...Bd6, which is far worse (Snake Benoni).
It is important to play Be2 Before Nf3 to avoid Black from trading his LSB for the Knight. You'd rather trade Bishops. So by developing the Bishop first, after Nf3, ...Bg4 can be answered with Nd2.

Personally, untill 2300ish FIDE, Benko is best response against d4 c4. It is very practical, and you learn a lot from it (How to play with active pieces, down a pawn, etc.)
After that, KID is a very good choice against d4 c4 setups.
My experience and opinion.

How is it your experience though if you're not 2300?
Benko is very cool though.
I have a friend who used it untill then.
But I can advocate that at 2000 FIDE elo it also starts to diminish in strength as it is hard to get winning chances.
There is so much wrong with this.
1.d4 c5?! 2.d5! Nf6 3.Nc3! Does not make ...b5 easier to play for Black. The moment Black plays ...a6, white can take the positional approach with a4, or depending on the line by Black, White can get a raging attack down the middle. It winds up either a Schmid Benoni or Half Benoni or Closed Benoni or Snake Benoni, all of which are good for White.
I would typically play a4 and take the positional approach with e4, Be2, Nf3, O-O, and then Nf3-d2-c4, assuming Black played ...d6 and not ...Bd6, which is far worse (Snake Benoni).
It is important to play Be2 Before Nf3 to avoid Black from trading his LSB for the Knight. You'd rather trade Bishops. So by developing the Bishop first, after Nf3, ...Bg4 can be answered with Nd2.
I mean a4 isn't so hard to meet. It is still making b4 square available and even after a5 push it's either weak or we can ingore it and play with pieces. Besides you can always go for e6 because d5 isn't supported by c4 pawn. And that isn't my personal belief but my coach's, a Fide Master.
Oh. Sorry, yeah, correct line is 3. Nc3 d6 4. e4 g6 5. Nf3 Bg7 6. Bb5+.
Sorry for the confusion.
Looking through those lines again, I actually tend to prefer 6. Bc4 or 6. a4, but 6. Bb5 is thematic and solid enough.