What would be a good investment now? I want no-frills Linux support, good CPU, lots of RAM, decent screen. If I'm actually working, I'm almost always docked, but when not, I would not mind a good battery as well. I want this primarily for personal use. I don't mind upgrading parts myself (if that's still possible), like getting a stronger SSD or something.

I used to own a T420 (and some other ThinkPads as well, but this one was used). It was an incredible investment at the time, used laptop price, build quality and feature on par with laptops 6, 7, 8 years younger. I wonder if this is still something you can get away with.