I administer around 35 users + 3 other markets, all around 20-30 users each. I was thinking of doing [this or that], but I’d like to run whatever in a lab environment before deploying. How should I build out this lab? I already have a handful of old computers laying around. I could use them for users’ Windows machines. What about Univention? the firewalls? etc…
You can just install UCS where you like (KVM, WMWare, VirtualBOx, AWS, home server, docker etc.) and connect the windows clients to the same network like your UCS.
This strongly depends on your usecase/scenario. If you use UCS@School you may have a look at Szenarien zum Einsatz von UCS@school - Organisation und Aufbau zentral verwalteter IT-Infrastrukturen für Schulen — Szenarien zum Einsatz von UCS@school first.