One common engine issue are worn valve stem seals, which can cause oil burning, and usually occur sometime after 100k. If you see grey smoke at the tailpipe on startup, its very possibly the issue. This is expensive to fix, though leaky valve seals have no critical detrimental effect other than increased oil consumption and at worst, accelerated wear on catalytic converters and O2 sensors.
Another known issue is crankbolt failure which is a big mess if it happens. The crankbolt was revised twice, the 3rd version having resolved the design problem. Versions 1 and 2 are prone to snapping. But its easy to fix, just install the updated bolt.
Also common are leaky valve cover gaskets, which while nothing major, are a pain to replace since the top of the engine needs to be removed, and therefore expensive if you're paying a mechanic.
Oh yeah, leaky front/rear cam seals and front crank seal are common repair issues as well. Often leaky cam seals will present as leaky valve covers since the oil leaks tend to follow the same drainage route.
Higher mileage Monteros usually need the following parts replaced, which consist of two separate jobs:
1) spark plugs, plug wires, valve cover gaskets/grommets
2) timing belt, tensioner and idlers, water pump, front/rear cam seals, front crank seal, updated crank bolt