well I don't see your use case anywhere to really speak to the "not so fast" but I've been running around 20 containers on an old refurbished Optiplex 7040, with 0 issues and a ton of headroom, for like $100.
and sure, "upgrading" is not existent, but I've thought of it this way: it's to learn on, and that itself is great for upgrading later, as I'll have a better idea of what I want and how to do it when I do get the big beefy server.