The cls450 is the head with the three dials on top. The da900 is the chassis with the negative stage lens, column etc. The cls450 on its own just makes light; you need an enlarger to mount it onto. In this case that's apparently a da900 (which I'm not familiar with).
In your place I'd ask the seller to demonstrate it to you so you can see that it works (and how) and then buy it in its complete, working state if you like it.
Okay, I just couldn't let this one go without more. I reviewed the full manual for the DA 900, and the Cls 450 is definitively NOT a colorhead Durst intended for this enlarger, probably because the DA 900 came and went before the Cls 450 arrived. That said, from the photo of the offered unit, it looks like someone went to a lot of trouble to custom mount the Cls 450 on a DA 900 chassis. Assuming you also have the power supply for the Cls 450, it could work. For me the question unanswered is how is the light from the colorhead being piped down to the negative carrier. If you look at the photo, you will see on the front panel just above where the carrier would fit in, a black panel with white lettering and a gray handle. This a hinged trap-door, opening to a cavity where condensers install to focus light from a bulb to the negative. Optionally, it can take a cold light head accessory. However, condensers will not work with this colorhead, so something has to fit in there to create an even illumination just above the negative. In the M800/700 models, this is one of three light diffusion boxes, the one used depending on the format if negative being enlarged. Since the same cold light adapter fits the M800 and the DA 900, you can assume that the M800 boxes fit the DA 900.
Negative carrier. The one for the DA 900 is unique. Other Durst carriers probably will not fit. I suspect that someone launched the Cls 450 modifications, expecting to pick up a DA 900 carrier in due course, and failed. Thus, the project went on the market. To put this set-up to use, someone is going to have to get crafty with design and 3-D printing, or get lucky with some vendor of Durst parts.
A bigger question not raised in the opening question: Does this DA 900 still have its original lenses? If not, I'd pass on it. the DA 900 is one of the several auto-focus enlargers Durst made. Durst was meticulous in crafting these units. The DA 900 holds two lenses; various focal length combos were offered. For each enlarger, the factory selected two specific lenses, then cut the focusing cams to the specifications of each individual lens. Once so matched, they are matched for all time. You can replace the lenses, but the auto-focus will not track the new lenses properly. Either lens mount can be manually focused a bit, but this was never intended to be a regular use and appears quite inconvenient. The factory matched lenses are usually identified by serial number on a plate at the bottom of the column. Making off with the original lenses (fairly common, as they were factory selected for maximum performance) leaves these auto-focus enlargers just a collection of parts, nearly valueless.
Hope some of this helps.
