Travel camera

Any of the models you've mentioned would work reasonably good enough, bar that old E-PM2 ( let's just scratch it off the list )

But as a note, do not just look at the camera, take a look at the very first setup ( which include such as lens, flash, accessories, and all that ) and system wise ( how is the depth of the system to grow into , options ) and for a beginner the UI ( User Interface ). UI is a multi-faceted and very personal thing. including such as ergonomics, control and control layout, control manipulations, and ease of handling ( for various task ). You need to read up on those products review and better yet got to physically handle the thing to see how it works. Thinkl that initial setup of gears as a single one, not just the camera.

That stated, financial cost of course made a big deal and here I guess the NEX-6 had an advantage , not only because of the fact you've stated about the deal but also for the fact that from field experience its 16MP sensor had shown low light prowness typically at least 2/3 to a stop superior to the others ( including its own siblings in some cases ). Whether that's useful to your exploration of night photography you have to decide for yourself. Your trip is yours and we really could only offer that much advice. You must look into how you are going to use the gear and what demand you are placing upon them. Technically and otherwise.

And aside from that, there is one specific area that's camera and system independent .. that's the post exposure development / processing need and the platform you would be using . In short, your PC and the software and of course how you are using them and making use of them. You need to develop your workflow. That can cost both financially and in effort and likely time consuming but if you want to progress its something you must handle.