Julien:

As I understand it, your enlarger probably is the same as an LPL/Saunders 670, in which case it isn't difficult to get things like new negative carriers. See, for example, this listing (and read through the offerings):

http://www.bhphotovideo.com/c/buy/Fi...899+4291304538

In addition, if it is a re-badged LPL/Saunders 670, it will have a colour head and will take up to 6 x7 negatives, which is also good.

Your enlarger is probably an ancestor of this current version:

http://www.adorama.com/SD670DXL.html

The B600 is similar to the current C700, in some ways, but IIRC it is limited to a maximum size of 6x6.

If both are in working condition, I would choose the Rollei.