I was trying to install Adobe Flash Player for Mac on a friend's 1988 iMac 20" Core 2 Duo 2.66GHz (MB324B/A) today but couldn't because it is running OS X 10.5.8. Flash Player will only install on OS X 10.6 - 10.9. So i checked that her iMac is capable of running OS X Mavericks, which it is, and then went to download it for free only to find that I can't. It would seem that she needs to first buy a later but intemediate version of OS X after which she will be able to upgrade to Mavericks for free.

My question is: what OS X version does she need to buy? Is it OS X Snow Leopard?

http://support.apple.com/kb/HT5842?viewlocale=en_US&locale=en_US

And you can actually install Mavericks 'clean', which avoids the need to do a pointless upgrade first... http://mashable.com/2013/10/23/clean-install-os-x-mavericks/

Thanks for that, Quinten.