Keep the Beseler 45MCRX and the Durst. The Durst has a dichro head on it I assume, which makes VC printing with a diffused light-source a snap, and the 45MCRX can do what ever the the 23C and the 67 can as well. The fact you have a cold light and condensor is good, and it is adaptable to a dichro head or maybe even the fancy new LED heads that are being sold over here at some point. I've found every 23C I've ever used (23C, CII, CIII, CIIIXL) to be rickety and imprecise in comparison to my 45M (original version of the MCRX)...

Can't speak to the B-600 as I've never used one, but they look like they're on the same level as a Beseler Printmaker, or other similarly priced entry-level condensor enlarger. The Durst isn't really a Durst, but it's still a Durst, if you get my drift, which means it's made to a higher standard than other entry-level equipment. Like I said, the fact it has a diffusion head on it is a plus for variety as well.

My 2.

Edit: the 45MCRX also has you covered out to 4x5 format, obviating the need to worry about getting another enlarger if you went up to LF. Other pluses include easy to source spare parts, and cheap lensboards.