The Mégane II Owners' Club

Hi all. This is my first time posting, and my first Renault. I'm wondering if anybody can help with this.
I bought a 2006 Megane Coupe Cabriolet 1.6 with 119,000 km on the clock.
I did a full history check on the car before I bought it and there has only been one owner from new.
It has been running fine except for it missing a couple of times first thing in the morning, then it's ok for the rest of the day.
I bought it from a reputable dealer and I trust him so I didn't check anything under the bonnet.
After the 5th day of owning it I got a " Oil Low" message so I topped up the oil . While I was topping up the oil I decided to top up the coolant, and when I opened the reservoir cap it was FULL of light brown creamy sludge. I asked a mechanic friend to have a look and he said, straight away that it's a leaking head gasket.
He unclipped a pipe (I'm guessing from the radiator) and it had some water/coolant in it and also the brown sludge.
He checked the oil cap and dipstick and they both look fine (just oil and no sludge).
The car was running fine (except for the cold start thing I mentioned) and the temperature gauge never went any further than two thirds up the dial.
I took it back to the dealer and he also suspected the head gasket has a leak so he has the car this week to work on it and (hopefully) sort it out.
My question is, have I bought a dud? or does a head gasket wear out like that? Should I be looking for my money back?
I'd appreciate any expert feedback on this.
Cheers

1.6 petrol at 119k 11years old head gaskwts can blow I would have expected to white oil as well but not always so the light brown sludge could be a failed attempt at a coolant seal with a product like rad weld usually just an age thing dont be too surprised if she needs a clutch soon either general rule of thumb for me is how much do I like the car and how long will I own it these two questions dictate how much I spend on repairs.