To summarize and add to what has been said (OK, *MY* spin on what was said, but still ...)
MBPs are light ans thin Retina models with no optical drive. They are meant to be opened only by Apple ... other than that and the warranty is almost certainly void as well as all future service refused by Apple.
15 in models are bigger than 13 in models, weigh a bit more, cost more. Decide if 13 inch is too small or 15 in is too heavy.
RAM is soldered to the logic board. Get as much as you can afford when you buy it. Especially if you will ever run a virtual machine package like VirtualBox or Parallels ... those dedicate a block of the physical RAM for the running VM image.
SSD is not intended to be upgraded by end-users. Maybe in the future you can get the right SSDs, maybe after all warranty is passed, maybe. Buy the SSD size you think you *will* need later.
Make a decision that fits both budget and needs.