I think you should know that next time before promising something, you need to make sure you have everything on your hand before proceed and do not try to salvage parts here and there which will only caused you more issues than you can handle on exceptional case.

Often times, we will resort to all kind of issues and most often, we will try to minimize the cost as much as possible. You can minimize anything unless you are perfectly sure that everything is working normally but we are human, we will tend to make mistakes here and there. But what is most important is to have backup plan or maybe support. Don't gamble on your decision as it's always going to stir things up rather than help you on the long run.

Since you are donating a pc, why not just delay it for a while for your sister or family first. Once you got yourself a new hard disk, then you can proceed with your good deed.