Hello,
The Durst M600 is a nice, compact enlarger that still performs well for its age. I am rather fond of this little Italian wonder. You can print a range of negatives up to 6x6 medium format and not have it take up a ton of space in a small darkroom of an apartment or small house. I have never used the AN glass inserts. Those would make it a truly universal negative carrier, but not generally needed for 35mm negatives when you have the proper Sixma 35 masks. Since I've only recently gathered proper enlarger lenses and lens boards for printing 6x6 negatives, I can not comment on how important it is to have the Sixcalo heat absorbing glass mentioned in the manual. Mine has never gotten hot enough that would seem to require it, but I picked one up about a year ago to compare results. It was not cheap and I have not gotten around to that project yet. Sadly, all Durst enlarger parts are harder to find, but some are still generally available.
I believe the M600's universal carrier is unique to that model, but the masks can fit some of the other models as well. I bought a box of bits years ago that had a negative carrier for the Durst M601 and the masks were the same size as the M600 versions. It would be best to study the manual and look for those specific parts. That online auction beast generally has a handful for sale and most are 35mm varieties. The 6x6 masks seem to be harder to find and the smaller formats are practically extinct examples, but they do pop up from time to time. (Be patient in your search!) As an example, there is a set of the 6x6 Sivopar 66 masks for $90 posted now for auction. The Sivopar is listed for the M601 and I know it is the same size as the Sixma 66 masks. I have one of each and they both fit. It probably means that one is the lower mask and one is the upper mask on some Durst negative carriers. I just don't know that information. (Please someone jump in here that knows what these Durst designations mean!)
If you don't already have a manual, check out this one found online:
http://www.jollinger.com/photo/cam-coll/manuals/enlargers/durst/Durst_M600.pdf
I hope that helped some! Welcome to the black art under the glow of a red light!