What do you mean by lightweight ? NixOS can uses every WM, every DE without any issues. It is lightweight because you know all packages you download... A friend of mine uses NixOs with a T440p...
Yes, NixOS does need quite a bit of RAM while rebuilding (~1GB) and takes lots of storage because it keeps older generations (similar to OS snapshots) around.
Otherwise NixOS isn't any more resource intensive than other OS. Anecdotal experience, but my NixOS system boots faster than Fedora Atomic with the same window manager and packages installed.
In any case, I've been using NixOS, Fedora Atomic and OpenSUSE MicroOS on my T480s without problems, so OP will be fine with any distro.