Or is this a bad idea?
Reading through !selfhosted, I think I have found a new hobby. I have an old laptop HP ProBook 450 G5 4WU81ES.
16gb ram, solid CPU, shitty integrated gpu, and only 256gb ssd. Barely enough for system and some apps. Battery life maybe 30min unplugged so I take it as an UPS.
So the question again is, can I have permanently plugged external hdd to use as extension for this purpose?
No reason why it wouldn’t work! Worst case the drive is pretty slow, but you do have an internal SSD so you can put OS and databases on the SSD and use the external drive for bulk storage.
I had a RPi set up that way for a couple years, worked fine as a simple NAS and Kodi for the TV!