I've owned my 126 Bis for more than 12 years now. It has been quite reliable in my ownership. That said, I've had to replace the ignition coil, drive shaft coupling and gear control linkage. All the aforementioned could have happened to an aircooled 126.
I agree with Xylaquin. In addition, you also need to keep an eye on the water temp gauge. Make sure you top up the radiator with coolant. Also make sure that the radiator fan works automatically (some Bis owners install a manual switch). I tend to change oil with a good quality oil like Mobile 1 on a regular basis regardless of usage/mileage. I think using good quality oil enhances the cooling process. With regards to the thermostat, I've not experienced any problems with it.
I would recommend anyone to buy a watercooled. It is easier to live with, and has all the mod cons, like a fantastic heating system, slightly more practical (hatchback has extra boot space and fold down rear seats). The engine is quieter, it has bigger wheels, so is smoother, rear wash/wipe, two speed fan, two speed wiper with intermittent function and is slightly faster (speed-wise and better able to cope with traffic) as well.