No need to change the plugs so early (rated 100k miles, and everybody that remove them earlier find them in like new condition at 80k+ miles).
Having a water pump changed at 20k miles is not a great history, its a potential overheat already.
The question of extended warranty is hard to answer conclusively. The car cost quite a chunk of money to keep working and many (minor) repairs quickly cost a lot. To what extent these will be covered is never clear though. Check that the warranty covers:
* Oil Leaks (from main pan, valve cover gasket, oil filter housing, main shaft bearing)
* Electronic failures (in particular airbag system failures are very common, even beyond the seat mat recall)
* clogged injectors
* dirty intake valves that provoke rough running
* water pump (already changed, but it may fail sooner than later anyway, and will probably be dead again @ ~100k).