imo 250gb is not enough at all, at least spec a 500gb or 1tb. I don't even have games on my 256gb machine and it's out of storage because of proprietary software bullshit.
Buying storage directly from framework is expensive though, I recommend just buying it from a local hardware store. The cheapest out of the sn550/570/580/750/850, crucial p5, kc2500 should do.
Also, 1x16gb will halve your memory bandwidth, get a 2x8gb kit