I didn't like the MTB geometry when it was new (for any distance, at least). BB was higher than I liked. Others may be happy with it.

Anyway It can work, but it seems a little strange to spend extra to build up something you wouldn't like as well as what you're used to.

The P-series you have linked looks perfectly fine for the job. Repack the bottom bracket and hubs if it's been a while. Put a hundred miles or more on it and check all the nuts & bolts for tightness - but a new bike would need that too.
You could put something like a 34-42 double on it, or maybe even a triple for not very much, especially if you have a local co-op. The front derailleur ought to be fine with that You know the bike, you know what noises it ought to make, it's comfortable - and that's huge.
At some point, there may be a hill you'll want to walk regardless of the low gear. But a 34/28 is about a 32 inch low. That's not bad for a tour without tent, stove, sleeping bag, etc..
You're talking about 600 miles roughly, yeah? You maybe do that in 3-6 months of commuting, and this should be less stressful on the bike.

For the other, If she's comfortable with the Atala, then why not look for something about the same size & geometry?
If used, there are some pretty solid older road bikes, like a Nishiki Olympic, a Miyata 110 or 112, various Univega, and some others that don't get a premium for the name plate. Total cost for one rearranged Peugeot and one "new" bike ought to be in the 2-4oo range, I'd think.

But if new ...
I've not ridden one, but the head tube is relaxed and the wheel base is on the long side. It looks cool, and a little retro, I think.
http://www.somafab.com/archives/product/buena-vista


The reliability is an interesting thing. My experience is not good with new bikes in the 4-5oo range. YMMV. Would put my money elsewhere.