I used to have old ThinkStation as a home server. Even older ones like S20 I have couple of laying around is still pretty capable system (I'm typing this on one) and as they've been CAD workstations and things like that when they were new many have 12+GB of RAM already. I got mine for free troguh a work contact, but they should be available via ebay or (preferably) your local version of it for pretty cheap.

Then you just need new drives and their prices have dropped too. 100€ is a bit of a stretch, but if you can get a whole computer from someone in the industry it should be possible. I have a few systems laying around I could get rid of for a case of beer or something, but shipping alone from here would eat up majority of your budget (if anyone is interested in x3550 m3 throw me a message, located in Finland, I might remember the model wrong but that's roughly in the ballpark).

Other than thinkstations I'd say you'll want a xeon CPU with at least 4 hyperthread cores, 16GB RAM and all the drives your budget has left. SSD for a boot drive(s) is nice to have, but spinning rust will get you there eventually.

Many rack mounted servers only accept SAS-drives which are a bit more expensive. Tower mounts generally use SATA so you can just throw in whatever you have laying around. The main concern is amount of RAM available. For older systems it might be a bit difficult to find suitable components, so more you have already in place the better. For VM server I think 16GB or above is fine for learning and it might be possible to shoehorn most of the stuff in even with 8GB. Performance will definetly take a hit with less RAM, but with that budget some compromises are necessary.

So, in short, with that budget it might be possible if you have a friend who has access to discarded workstations or happen to stumble in a good deal with local companies. It'll require some compromises and/or actively hunting for parts and with old hardware there's always possibility of failure so plan accordingly.