Where were we? Ah yes, the Megane...
As mentioned, head gaskets go. This is often brought on by a furred up water system, which *needs* rust-inhibitive coolant. However...
1) the official drain/replace schedule is 72k miles
2) because the cars tend not to use water, and the anti-freezy stuff in the coolant passes garage anti-freezy tests, people seldom consider their liquids before the 72k service mark
3) the effectiveness of rust-inhibiting agent decreases over the years
...so, the car's arteries clog up with rust even though it's been serviced according to schedule, leading to temperature differentials within the engine block, leading to all sorts of HG failure and cylinder head warpage nasties.
Radflush it and fill with nice methane
-based coolant with added anti-corrosive, and if you're mixing it yourself use deionised water as tap will severely reduce the effectiveness of the anti-corrosive. Refilling is a bugger, btw: it's Airlock City.
And, obviously, find out when the timing belt was last changed. If it's original (which is quite possible) then change it immediately.
But, generally, the early Meganes are sound and reliable cars (unlike their niggly younger brethren).