The problems with the 2.5L V6 engines in the V6 626 and the MX-6 and Ford Probe include:
* Leaking valve cover gaskets - just look for oil in the front valve cover gaskets (where the plug wires go). Not TOO expensive to replace, but you'll need new plugs and wires too if you see it.
* Distributor - sometimes goes on it, can be VERY expensive if you get it from Ford. More common on the earlier models, not sure about the 97-99, I think it's been fixed.
* Timing belt/Water Pump - needs to be replaced at 70k-90k, BUT remember that this V6 engine is non-interference, meaning that if the timing belt breaks, the top of the piston travel will not hit the bottom of the valve travel, so it would just leave you stranded on the road.
Other than that, it's a NICE engine, very flat torque available at all RPM. With the manual transmission, the 626 is a FAST vehicle, faster than your average Camry or Accord.