This can all sound quite daunting for a new Mac user but do it right first time and you'll have(hopefully) years of trouble free music making.
to distill it down...
256gb system drive is TINY. You'll get away with it if the machine will ONLY be used to run PT and NOTHING else.
The internal Flash Storage Apple use is SUPER fast AND AFAIK, soldered to the mother board so not an easy upgrade down the track.
Whether you go 512 or 1tb for the system drive, an external drive of some sort is DEFINITELY a good idea especially if (as you mentioned) you work with large sample libraries.
Lastly, ask yourself do you really need a laptop? You'll get much better "bang for Buck" with a desktop, especially as for as ram goes! If you're running any sort of VI that utilises samples, you need lots of ram, 16gb is OK, 32gb is better.
Listen to the guys on this forum, they've likely made all the mistakes and have learned from experience, as a Mac newbie it may be tempting to chuck your hands in the air and say, "F*ck it, she'll be right" but believe me, you'll open up a world of frustration! PT works great IF you follow the buying guide and optimisations found on this forum.
For what it's worth, I recently built a new MacBook Pro system for a client, he had the money to spend and went to 1tb system drive, after everything was installed there really wasn't a lot of space left on the internal flash storage, as already mentioned, it's a good idea to ALWAYs leave a good 25% of any drive as free space and back up/clone often, as far as external storage goes I used an OWC Thunderbolt dock (already mentioned, google Macsales.com) and a Lacie external 4TB thunderbolt/USB3 enclosure partitioned into seperate volumes for Samples, audio and anything else that wasn't an app and could be stored externally, trust me, don't skimp on this part, it's VITAL to smooth operation,
Finally, I have to agree with Daryl re the touch bar on the new MBPro, apart from the fingerprint recognition ( which is great when installing a bunch of stuff and not having to repeatedly type your password
) it's not of much use for audio.
Hope this helps.