thermostat?

[John Larson]

You asked 

"How difficult?"

The thermostat is behind the timing belt.  It "can" be accessed with the 
belt in place, but it's pretty difficult.  If the car is anywhere near a 
multiple of 70K miles, or if the belt is older than 70K miles or 5 
years, I'd do the belt and rollers, as well as the water pump and any 
oil seals that seem iffy.  The pump's the weak link.  You need a couple 
of special tools and the manual.  Be sure to use an approved coolant.  John