You're absolutely right, this kind of business with refurbished business laptops is quite common worldwide. While affordability and durability are key advantages, ensuring the security of these devices remains a crucial consideration for me.
Easy: wipe them and install whatever Linux distribution you prefer. I'd never use any pre-installed operating system, and prefer to buy my laptops empty.
If you're really paranoid or security conscious, see if the firmware isn't the last version or if it is the last available version, maybe you can downgrade it temporarily. Any refurbished laptop that came to me so far didn't have the latest bios/firmware. Flash the latest version, wipe the storage as LoyceV said and install the OS of your desire fresh. That's safe enough for me, I'm likely not a target for three letter agencies and bios malware is probably quite rare and/or more something for specifically targeted attacks.
Another thing to check: power consumption. The less, the better. It's quiet, saves money, attracts less dust and stays colder. Whenever I buy a laptop, I first search the "TDP" for the CPU. The lower the better.
I'm kind of in the same boat as I rarely need any of my laptops for computation and energy heavy stuff. Keeping the cooling section clean gives you a longer life for your device. Dust is the enemy.
I never remove my laptop battery, and after 8 years it's at 70%. If I'd disconnect it, I can't use it when I need it (for instance when I unplug the laptop).
Well, it depends on the level of wear the supplied battery of the refurbished laptop already had. You can be lucky or not so much. That's also dependent on the refurbisher and availability of good batteries for your laptop model.
I rarely need any of my laptops in a truely mobile situation with no power plug. But that's me and YMMV.