In the US, the division of Durst enlargers between distributers is that Jobo USA handles 4x5 and smaller and Durst Pro USA handles the larger models.
Jobo USA has a document with the dates for which Durst guarantees product support:
http://www.jobo-usa.com/bulletins/b030.htm. According to the document, Durst has typically supported discontinued products for ten years. The L1200 / CLS 501 / EST 500 A are listed as "available" with no announced termination of support.
Your Optimo CL is supposed to be supported to 2006, so if you ever want to fix it, now is the time. What is wrong with it?
My idea on buying a L1200 with CLS 501 head is that all of the components are straightforward and could, if necessary many years from now, be made by a component machinist. The power supply is basically a transformer with 24 V output. The dichroic filters could be replaced with generic ones, though I can't imagine that I would ever wear them out. The ELC lamp seems to be very common and used in many products of many brands. If you want a condenser head, be careful with the condensers because they would be difficult to duplicate. The L1200 seems well made and not likely to break unless abused.
I would be more concerned with sophisticated electronics, e.g., autofocus or color regulation. If electronics like this broke, it might be difficult to diagnose, and impossible to fix if some IC were no longer available.
Ultimately, you have to decide whether to buy the enlarger that seems best to you on its merits, or if that enlarger is a less common brand, to buy a more common brand, guessing that manufacturer will be in business longer, or that used parts will be easier to find.
It must be very tough for all enlarger manufacturers. There are so many excellent used ones on the market. For the price of a new L1200 you could buy two, and store one as a source of spare parts.