I have fixed up and stress tested six ThinkPad E490. Base price range is $110-180 before shipping depending on hardware options. Cheaper for low res screen or if the case has more cosmetic blemishes, stuff that doesn't matter for a server. I'm fixing up these machines as an eco friendly hobby.

This model is particularly good for nodes because it supports two internal SSDs which is essential to maximize lighting database reliability and safety. They can also use up to 32GB RAM, although I tend to use 16GB RAM which is plenty good for a node. This is far more bang for the buck than raspberry pis. DM if you're interested.