Lenovo ThinkPads work very nicely with Linux, and there's a large second-hand market. The T and X series are especially great I find.
The usual advice about avoiding soldered RAM holds in general, but right now used laptops are being bought just for stripping RAM. So I think putting up with soldered RAM in second hand devices (I'd go for at least 32G) can be a smart move because it may be a better deal (and often a smaller form factor).
Off-topic, but isn't Fedora technically upstream from RHEL?