http://www.crushmy307.co.uk/
They have been plagued by unreliability. I have 2 306s (1.6 petrol and 2.0 HDI) as I know the 306 like the back of my hand. The 307 is a lot more modern but has got a very bad rep. TBH I think a lot of it is due to the type of owner and the current standards. Back when cars like the 306 were new you expected problems and the cars were not as technical. Now the people who own a 306 are more likely to be into DIY repairs.
In contrast the 307 was up against lots of german and japanese cars and the owners are not the type to work on them.
Mum has a 1.4 petrol and it recently cost her £700 for the MOT (gulp) due to little things like the indicator stalk (common fault) and exhaust which she had to get done at a garage as she didn’t mention them to me when she was down here on holiday.
The HDI engines do have dual mass fly wheel problems – I think you can replace them with normal type. Back when the HDI came out a lot of people feared them due to the high parts cost but there is nothing on HDI I can’t fix for very little by shopping around or getting used parts.
The fly wheel and any DPF stuff would be the things I checkout most on a car I was interest in (the peugeots have a chemical that is added to the fuel to burn off DPF deposits).