Doing all the hoses on an aging engine is always a good idea, and is good insurance, even when they're not leaking. My suggestion is get the cooling system under control first, and then tackle the idle.

Eddy's suggestion of head bolts is not for the idle, but purely because of the heat you may not have even noticed, when the coolant was low and the gauge sender was above the waterline. You certainly want to keep that head clamped on good.

Here's the factory service manual link, though you can find it in the mentioned stickied thread at the top of the list...
http://www.mitsubishilinks.com/fsm/montero_fsm_1989_enginechassisbody.zip

Phil