It sounds like you're probably aware, but the most likely reason for the blown head gasket is overheating, which is usually caused by poor cooling system maintenance. The best money you can put into making your Montero reliable is to go through the cooling system completely - if there is any sign of rust in the system do a Thermocure treatment on it, then flush it out several times. Remove the radiator and flush it out several times, replace all the hoses (radiator, throttle body, heater, etc), thermostat and radiator cap (use OEM for the stat and cap). Check your fan clutch and auxiliary fan for proper operation. I'm assuming you'll be replacing all the timing belt/water pump service components (using only an OEM water pump and tensioner) while the head is off, and it would be a good idea to replace the o-rings and gaskets in the coolant pipes as well.
