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?
Go for it. the specs on the laptop are good enough to run plenty of stuff as a server. As long as the drive can be mounted you should be good. Your backups should be somewhere else.