Patrick has an old 2008 iMac and he's looking to repurpose it. He wants to install a new SSD, more RAM, and try and get it up to Catalina. Leo says it's possible, but he says that High Sierra is good enough. Leo says to watch the videos at iFixIt.com on how to upgrade on an iMac to see if he's up for it, because it's challenging, including removing the LCD screen to get to it.
What Linux should he run? Leo says there are plenty, including Majaro (Leo's favorite), but he isn't up to speed on how to convert a mac to Linux. So do some research on Google. Also, check out the Linux SubReddit and ask the community what to be aware of.
The chatroom suggests ElementaryOS.