how to reinstall Mac OS after erasing -- but getting error message

I'm very non-tech-y but I felt that, if I followed instructions from the internet, I could deal with this issue myself. Sadly, I'm not sure any of it worked. So, to explain, I have a very old (2009) iMac (desktop). I purchased a new one so want to give my old one to a family member. But, I wanted to erase any sensitive data on it first. So, I did an erase through disc utility (which seemed to have worked). Now, seems like I need to reinstall Mac OS. I've tried to do so through OS X utility --> reinstall OS X. I get a pop up that says my computer's eligibility will need to be confirmed, then I select the Mac HD disk (only option as the recovery disk one is locked). This brings me to sign in with my apple ID. Then I get a message saying "this item is temporarily unavailable. try again later". So, I thought maybe my original set up Mac OS X install DVD might work . . . but my computer sounds like it might accept it . . . then nothing happens and it spits it right back out. Is my computer now pooched, or can this be rectified and, if so, how? My 8 year old grandson much appreciates any help as he's been waiting/hoping for this computer for a few weeks now.