Take the lens that best matches your vision for your subject matter. Pressed to one lens, I would take the 12-40. As for bodies, an E-M1. Runners up would be a GX7 or E-M5. Go with what works for you, or with what you have. I suspect that you could make do with just about anything if you had to.
Location, location, location.
I recently took a trip to Rome and Florence and elected to take the GM1 with the 12-32 and a PL 25 f/1.4 in place of my DSLR 2-3 lens kit. I had some advice from a Rome local who reassured me that the 12-32 was a pretty good all-around choice. It worked out fine as I pretty much left the 12-32 on 90% of the time. The 12 was usually wide enough and the 32 was usually long enough. But the simplicity and easy transport of a single camera/lens was thoroughly enjoyable. The caveat is that these were locales begging for wide rather than long focal lengths. A 12-40 would have been ideal. A 14-140 (or 150) might have gotten a few other shots that I didn't get - but in other locales would have been essential.
Limited to only one lens and body from the kit I already own the logical lens kit would be the 12-50mm mounted on an E-M5 body. It's not (by far!) my favorite lens but it does a bit of everything (macro, wide angle, short telephoto) and is weatherproof.
Since I am not always logical however, and having had my fill of the 12-50mm on a short trip last fall, I think I would take the E-M5 and my Panasonic 20mm. Certainly not great for wide vistas or portraits, the 'short normal' 20mm nevertheless is a proven 'can do' lens for me and while I know I'd miss a lot of 'touristy' shots the shots that I came back with would bury whatever I could do with the 12-50mm lens. The 'normal' range (say 38mm to 52mm, give or take) of lenses has been a standard fall back for photographers for decades and it seems to work for me quite well.
Of course one reason for getting an interchangeable lens camera is to have the ability to change lenses so I don't plan on heading off to the Himalayas carrying an E-M5, 20mm, toothbrush and spare socks anytime soon .
This morning I ordered a 14-150mm 'super zoom' Olympus - I'm headed to a soiree soon at which I don't want to swap lenses or carry a bag or second body and from which the image quality of the photos isn't the 'end all - be all' of the day. That might turn out to be the 'one lens fits all' solution for vacation snapshots but as it's not in my kit yet I don't think it fair to claim that I would take it.
The 20mm though - that lens I can depend on. I've been tempted by the new Olympus 25mm f/1.8 but my affection for the 20mm has prevented me from ordering one. That and the fact that the Princess of the Exchequer would have my hide for buying another lens in a focal length that I already have covered.
I did a few months in Asia with a fixed 35mm film camera (Yaschica T4 super D) and didn't miss my heavy Nikon kit that I left back home. Going small and light can work great, and if that's tempting, then just take whatever body you have with the P20, O17 1.8, or the PL 15 (or one of the 25s... Just bring something fast and versatile), but if you want a bit more, then probably a mid range zoom would be best (12-35 or 12-40) mounted on whatever your favorite current body with an EVF is.
Much as I hate to admit it the 14-140 is probably the most useful, if you add the 7-14 you are pretty much set for the daytime. Add the 20 or 17 and the 45 you have all you need, don't forget the batteries.
I'm going on vacation next week and was, too, looking for a travel lens. I settled on Panasonic 14-42 II. I was testing this lens over the weekend on E-M10 body and was thoroughly impressed by sharpness and color at both wide and long end. I'll be taking 14-42 II, PL25, P14 and P45-175. Not sure if I need telephoto lens (Mexico) but it's lightweight enough to have just in case. Also, I will most likely end up using either P14 or my RX100 for low light. I am afraid PL25's DoF is too shallow for travel and and the angle is too narrow. But it's my favorite lens so I always take it with me as it guarantees keepers every time I use it. Panasonic 12-35 is an interesting choice for a travel lens but 32mm is too limiting, IMO and I am not sure if f2.8 will cut it in low light. Oly 12-40 is way too big/heavy, kind of defeats the purpose of m43 altogether for me. Definitely not for travel.
I see that you have the PL25, and that's a nice normal lens for travel. The kit lens is also decent enough for general day time use.
As m43 kits are generally small and light, you can get away with 2-3 prime lenses and even a tele or ultra wide zoom. I would recommend the P7-14 and any of the current primes in the market depending on your preference.
My current travel kit is the ancient GF1 x 2, P7-14, P14 & P20.