I'd just buy a 16GB kit of DDR4 SDRAM @ 2133 Mhz. You can go up to 32GB total. Newegg's got a good price on the G.Skill kit. Just bought a 702VM last week and I'm thinking about getting it.
Note about removing the bottom compartment. I began prying it open from the hinge. I had to use medium force to get the first latch to pop, but after that it went fine. I threw a Samsung 950 Pro in there. This thing is blazing.