So essentially you want a cheap PC for you parents who just brows the web and use excel sometimes. and you friend has an old pc. And as for the pi, I think it sounds like your best bet, as long as you have a monitor to hook it up to. I don't think you can get the excel app, but office 365 online should be fine. If they can't do the online one, then probably just get a cheap windows pc, because I'm not sure how usable a desktop as old as that can run anymore.