My old laptop died lately and I want to have it fixed by a repair-shop.

Now first I want to clean the hard disk (via an external 2,5 inch casing).

I managed to connect the drive via USB in order to backup files and remove personal data.

Now I'm about to delete old partitions.

However, it might be useful for the repair-guy (or woman!) to have access to the OS. Or at least have a proper boot loader. 
I'm not familiar with preparing the disk for a new OS install.

A. can I simply delete all partitions (i'm using ubuntu and Gparted)? Is there a risk of deleting / formatting too much?

B. can I install an OS on this harddrive, then disconnect it from my working laptop and put it back into the old laptop?

EDIT: it seems Mint is versatile enough to move between different machines ([How could I install Linux to the hard drive of a different computer (one that won't boot from cd)](https://superuser.com/questions/383820/how-could-i-install-linux-to-the-hard-drive-of-a-different-computer-one-that-wo))