Do you have reason to suspect that the heads have a problem? If not, taking it to a shop to have the heads removed and checked is a very expensive waste of time. Unless you have bad numbers on a compression check or oil and water mixing, there's no need to pull the heads, and certainly no need to plan on replacing them.

First priority is to get your cooling system corrected. Pressure test the cooling system to see if there are any leaks and correct anything you find. If the original radiator is still in the vehicle, use it. It's far better quality than most of the aftermarket radiators you can buy. Take it out and take it to a radiator shop to have it rodded and cleaned. Plan on replacing the radiator cap and thermostat with original Mitsubishi parts - they're far more reliable than the garbage your local parts store carries.

If you don't know when the timing belt was last replaced, you need to plan on doing that in the very near future. You will want to replace the water pump, timing belt tensioner (use a Mitsubishi part only) and pulley, and engine drive belts at the same time. You also need to check that the engine has the updated crankshaft pulley bolt and replace it if it doesn't. It needs to be torqued to the updated torque spec as well.

There are lots of posts here regarding the typical maintenance jobs for your Montero. Do a little searching and reading to familiarize yourself with what's necessary and important.