The D II can do everything between 16mm and 4x5 - as long as you have the right negative carriers, lensboards and condensors.

The Beseler will do everything between 16mm and 6x9cm - as long as you have the right negative carriers. In most cases, you won't need different lensboards, as long as your lenses have the same thread sizes. The condensors that come with a condensor head should handle all those sizes, as long as you adjust that upper bellows appropriately.

You cannot really modify a Beseler 23 series enlarger to use 4x5.

Accessories and parts for both the Beseler and the Omega DII should be relatively easy to find.

I just went from a Beseler 67 series enlarger (with both condensor and colour heads) to an Omega D5 (with both multigrade and condensor heads), but I'm keeping my Beseler enlarger in storage, because if I can ever move again to a dedicated darkroom, I'll use both.

In your case, I'd keep using the C700 for now, but keep and store one of the others, until you can either figure out how to use it in your smaller space, or get a bigger space.

Matt