Im looking at building a Glock in .40 that is going to be suppressed. I have an extra Glock 23 sitting around that has no barrel, so I pretty much only need recommendations on which barrel and suppressor.

I have an AAC can on a walther p22, but don't have any experience with cans on bigger bore pistols.

Check over at the armory section of this site.  There is a C3 fourm I think.  

The .40 would not be as easy to suppress as a .45   The .45 round is already subsonic so you already have that going for you.  But I personally just don't like the .40 cal  I think it is a waste.

Oh ok, I'll just throw all the .40s away and buy .45s

I don't think that he is saying "toss the .40 guns."  Since non +p .45 is subsonic, you will derive more benefit from the supressor than you with with a ultra-sonic round.  You will get about the same performance as you would with a non-suppressed .45 as you will after you suppress it, and you will get the increased sound reduction that you would by using subsonic ammo.  

Also, I believe that .40 has it's merits, but I have stopped buying, and sold all of my .40 guns because I can't afford to stock so many types of ammo.  9mm, 5.56, and 12 gauge are the only calibers I will be buying en masse.  I keep .45 around for my house gun, but that is all I use it for.  

I'd decided on an evo-9 suppressor, but didn't have a suitable 9mm for the can.  I did have a 23, though.  I ordered an extended, and threaded conversion barrel for the 23, and only had to buy Glock 19 magazines to shoot 9mm.  

Now I have a .40 Glock, and a 9mm Glock, with about 1 minute to change calibers.  With a 9mm suppressor, I can mount it on a SBR 9mm in the future.  

Lots of 9mm subsonic out there, and the general concensus is that 9 is easier to suppress than larger calibers.