Differences between D200 and D200 companion

[Axel]

Slave same like master, but without remote control / network card

So buying a used D200 to use as a slave (that may be found a bit cheaper than a new companion), should work just fine to make up a D400?

I'm also pondering a companion for my D200 but I may wait a few months just to see what Devialet is up to.

Using a D200 master as a slave(companion) will work fine and it gives you wider options should you decide to sell one or both. Also you will have two remotes.