Download all windows .ISO that you require for your reformatting transactions.
Example: Windows xp, 7, 8.1, 10 32&64 bit
Buy 4 usb sticks or flash drives with aleast 4 gb on it (8gb preferable)
Use rufus to install each iso to each stick.
Buy a key chain to attach or hook all of them in your belt holder. (dont forget to label them this is very important)
Congrats, you are now a walking reformatter man!
Add a portable external hdd later for backup purposes before formatting, as well as its where you save all programs/games to be installed later if the owner wishes it, and ofcourse at an additional price like 50 pesos per program (life mircrosoft office or etc)
Good luck.