Disclaimer - I don’t DIY much but have owned both Buchla and Serge over the years.
I think either could make an interesting system in that size case especially if you made your own design faceplate(s) and packed the PCBs in there a bit!
Do you want to interface it with your Euro system? If so Serge might be easier as it is readily available (I think) with 3.5mm jacks. Buchla - you’d have to build a converter box/panel for the bananas. Also keep in mind that Buchla CV is positive only which again makes interfacing a bit harder. Buchla can also be a bit fussy about triggers as well.
Your starting point should maybe be whether you are drawn towards the Serge or Buchla patching style, philosophy and sound. They’re pretty different. The separation of CV and audio in Buchla certainly makes patching quite different to the everything (easily) goes everywhere patching of Serge. Do you want building block type modules or more complex multi-featured ones? Do you want LPGs?
Not sure if that’s much help!