1. d4 c5!?

Any advice about playing this as Black?

1.d4 c5 2.d5 f5 is the Clarendon Court Defence, also known as the Dutch-Benoni Defense.  It is fairly obscure, so it has some surprise value.  But I think most strong players consider it positionally unsound.  I have played it without much success.  You would probably be much better off as black playing the King's Indian Defence, which has a similar feel, but without nearly as much risk.

GM Jonathan Levitt has a video about the Clarendon Court Defense (I have it), but I have yet to see a book that advocates this strange opening.  There is a chess.com goup devoted to this opening.

Please let us know if you try it out.

By the way, 1.d4 c5 also is the bginning of the "Old Benoni Defense", but I don't know much about that.

The benoni family is rather large : schmid , czech , modern, old ... are there more ? The first time Fischer ever beat Spassky was Fischer playing black in a modern benoni , Tal used to play this opening as well . 

Most Benoni-type openings begin with 1.d4 Nf6, so I assumed the OP was interested in one of the two openings that I mentioned in my previous post.  But of course there are several ways to transpose between the various members of the Benoni family.

I forgot the franco-benoni !?  yikes !  how many benoni siblings are there ?!   ECO classifies 1d4 c5  as A43 , old benoni defense but ofcourse at this early stage it can transpose to other benonis or even something completely different . 

Reb, everyone I have ever asked has warned me to stay away from the Benoni openings (as black).  "Too risky", "must be an expert to survive", "Modern Benoni has been refuted".  Do you think this is good advice for someone with a rating in the range 1600-1800 USCF?

It is definitely risky !  You wont have many draws with it for sure but I had good results with it except when I played players over 2200 .... it was definitely NOT a good choice against masters ! I no longer play it regularly and favor the gruenfeld ( when I need to win ) and slav when 2 results are ok ... ( losing is never ok ofcourse  ) . I always preferred the modern benoni and now rarely play it except online . I also rarely allow white to play the early f4 lines against it which means I play it only after an early 3 Nf3 by white .... this greatly reduces your work load and the risk as the early f4 lines are very good for white ... 

Thank you for taking the time to explain this!  This really helps.  

What about the Hromodka system (3...d6), keeping the e-file closed? Black can still transpose to a Benko, a modern Benoni, or the even the Czech. Isn't this move order how the Benoni used to be played?

I meant as a follow up to d4 Nf6 c4 c5 d5

Switch to the KID, Nimzo and Queen's Indian, or Dutch.  You want to generate winning chances without becoming too positionally weak.  

Lower level players usually play 2.dxc4? (2...e6!? puts white in a very awkward position if he wants to retain the pawn) which makes it my personal favorite OTB response to d4. If white plays correctly (2.d5) you get the Benoni which is very complex positionally but is very fun to play. 2. Nc3!? puts black in a weird position with an overextended pawn structure after 2...cxd4. In the main line, white has pawns on e4, d5, and c4, black has pawns on e3, d4, and c5. However, 2.d5 is better met with 2...e6 3. c4 exd5 4.cxd5 d6. Then black should fianchetto the king's Bishop to support a later queenside attack. Typically black pawnstorms in hopes of creating a passed pawn. Make sure to keep the d6 pawn defended or your position will collapse. Risky, but fun.

c5 as a response to 1d4  is a suboptimal move.By the way 2dxc5 is good too.