A:AnswerI'd suggest noting the model number(s) and compare. There are only two 2-TB 27" IMACs. One from 2015 and the 'Latest' 2017 IMAC.
The price fluctuates... I bought the 2015 - 2TB model and saved over $500. If cost isn't a concern, buy the new one, but get the 2TB Hard Drive.
A:AnswerYes you can and it's best you do it yourself. Best Buy doesn't carry the best ram and Apple charges a arm and a leg. Takes 5 minutes to do hardest part is choosing the correct ram and opening the package but a quick internet search will help you with that! Good luck