Erik here:

"An effective watchdog is independent from the main system."
"A really effective watchdog cannot use the CPU's clock, which may fail. A bad solder joint on the crystal, poor design that doesn't work well over temperature extremes, or numerous other problems can shut down the oscillator. No WDT internal to the CPU is really safe."
and more comments found in the same article at

http://www.embedded.com/story/OEG20030115S0042

I think using the wdt with PLCK as its oscillator could be good enough for this specific application I have. For future projects I'll keep in mind this power down modes and wdt interrrupts.

What do you think!
Thanks