RE: changing spark plugs
You need to get a new intake plenum gasket from your local parts store. Also, you should only use NGK Laser Iridium plugs. The will cost $12-$15, they will last for 100k miles. It takes about 1/2 hour to remove the plenum, so the entire sparkplug change job will take about 2 hours. When you are in there you might as well change the spark plug wires too. You won't see many wires that can go 10 years and 200k miles.
'90 Montero (sold-sadly)
'95 Montero LS (166k) - my car
'03 Montero Limited (36k) - wife's car