Gooooood loord, you're moving fast, just catching up with the thread.
Papa, go for largest memory available, it's soldered onto the logical board. You can potentially upgrade your SSD, but I guess that might void the warranty.
http://www.everymac.com/systems/apple/macbook_pro/macbook-pro-retina-display-faq/macbook-pro-retina-display-how-to-upgrade-ssd-storage.html
I strongly suggest getting the Apple Care extended warranty, well worth it, but buy separate from Amazon or eBay as you can find it half price. It recently happened to me, my MacBook Pro has memory and battery problems and to fix it they need to change the logic board plus the top case because the battery is built into the top case. Total cost if I wouldn't have had warranty was £623, see bellow: