Quick tip for purchasing used (or new?) MB/MBP/MBA required

I never used any laptop before aside from Office for school (then an old MB and later a tiny HP book). Now I am looking forward to a long South America vacation, and I am considering a nice travel video, Brandon Li style. Plan to import UHD footage to an external drive (where I create the library), organize the material, pre-synch audio asf. Actual edit on my 5k iMac at home.

So the book should be able to play back UHD H.264 at least once with FCPX (to check focus), I can do everything else with proxies. Resolution should be at least FullHD. Size? I think, the smaller the better (or am I wrong there? Think I'd prefer a smaller Retina screen, but lack any experience there). Prize? Well, as it doesn't serve any other purpose (at least what I can think of right now, usually the iPhone suffices for E-Mails and the like), as low as possible, but Im prepared for up to 1200 €, in case that divides junk from actually useable machines. Any recommendations?

I edit half-hour shows daily at my TV station, lots of mO2 in Motion, 4K H.264 exclusively, on a 2005 15” rMBP with very little problem. Never use proxies. Very fast external RAID. So you don’t need mega-super-powered laptop.

Thanks.
This definitely falls into the price range
In other words, every MB will do, and I could as well buy a new MBA?

Let me say, I've done a test with a single SSD, T'bolt 1, 500 GB external drive. It was not fast enough for 4K multicam or major 4K timelines. My point is a laptop can do it, but only with fast enough storage.

I see. I'll take care of that.