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?
Is the internal drive replaceable? That might be a better option. Alternatively, 256gb is more than enough to install Linux (or proxmox) and serve a lot of useful apps. You only need a ton of space if you are planning on storing media.