Used Mac (what production year is too old)

I posted a few weeks ago regarding information about getting A iMac for ProTools 11 or ProTools 12. I've decided to take the used route in order to get everything I need within my budget. I've been in search for a 27 inch i7 quad core with at least 16 gigs of RAM and a 1 TB serial drive. My question is what's the earliest year that I should feel safe purchasing. I found a couple that were produced in 2011 that fit my budget with all the specifications that I listed above. But I'm not sure if anything has changed from then till now in terms of adding extra RAM to the older iMac's if I wanted to.