Is it a windows 10 machine or Ubuntu? If it's a Windows 10 I'd upgrade to Pro (normally £120 using the official store) and turn bitlocker on if it's Ubuntu then you don't need to worry about it as your disk drive will already be encrypted. They shouldn't need your passwords to do tests on the components. Also Just deleting files doesn't make them disappear. If you want to try and make them disappear for good then use a tool like Erasure.