Hello Patrick. As said, Durst is still selling small parts like negative masks and diffusers for these enlargers. ASK BEFORE BUYING with which diffuser your M670 comes with. You may want to stock the 6x7 conversion kit for future use, as long as it is still available.
I wonder how some previous poster could suggest a L1200 instead of it, as it is an incredibly bigger, taller, more espensive and heavier unit. Perhaps he never actually saw one.
Coming to topic, I also had an M600 in the past, and actually I'm not crying for it. Yes it was solid and worked good, but I HATED to have the bottom of the condenser as (fixed) upper negative glass. I think you will find the M670 head "lighter", built with thinnier metal, but still extremely sturdy and easier to use. Some details, however, are even better: like the rectangular-sized column, which is much harder and rigid and beats hands down the itty-bitty M600's tubular column.
In few words: I can hardly imagine you regretting the switch from the M600 to the M670.