I'm an old tourist who did some racing BITD mainly to get in shape to go on tours.
Old French bikes, like your Mercier 100 or 200 (I'm not sure which one you have) typically have very good geometry for touring with long wheelbases and clearance for fat tires. I've toured on a Peugeot and a Motobecane of that generation. Both were fine touring bikes as they took fat tires and fenders and had sufficiently long chainstays.
The French made a lot of different cranks that were very suitable for touring and in fact likely pioneered the idea of running smaller chainrings to get decent gearing ranges. The stronglight 99 used an 86 bcd chainring so you can go as low as 28; the TA 50.4 bcd crank can go as low as 26. The stronglight 49 on your bike can be modded to run 50.4 bcd rings if you want to improve your climbing gears. If you tour in the flatlands around Chicago, that won't matter much. If you decide to go somewhere more scenic like SW Wisconsin, I'd mod the gearing.
The Mafac racers were state of the art BTD and are still fine center pulls. If you haven't done so already, I'd invest in some good quality brake shoes (kool stop makes replacement pads for those racers).
The simplex derailleurs are frequently bashed and maligned. The decision to use that much plastic (delrin if you like) was a mistake and simplex had to have known that early on. That said, the simplex RDs hold up better typically than the shifters are the front derailleurs. Plus they shift well due to the double springs. In fact they shifted better than pretty much better than any RD of its generation other than the suntour which had a patent on the slant parallogram and certainly shifted better than those fancy Italian RDs the OP no doubt raced on BITD. Plus the simplex RDs typically can handle quite a large freewheel.
That said I'd dump that RD for a suntour. It will shift better and I'd trust an old all metal RD over one that has a lot of really, really old plastic. As long as you plan on touring on an old bike, you might as well go totally retrograde and replace that plastic saddle with a leather one. They're comfy for long rides at lower speeds which is what most tourists do.
Oh yeah, I set up my '73 Motobecane Grand Record pretty much in that fashion. I dumped the fancy Italian racing derailleur that it came stock with for a suntour, replaced the brake pads for kool stops, and I'm running generous gearing (50/34 rings and a 14-28 5 speed freewheel). The bike fits stout Schwalbe mondial 700 x 35c tires easily. It's a fine all roads bike and would make a very decent touring bike as well.
