Are Espaces as Unreliable as the reviews say?
Sorry, but you did ask.
Of course it is true that those who experience problems are far more likely to post reviews than those who don't. I very much doubt that I would ever have joined this forum had I not had problems with my Espace. If it's any comfort I am led to believe that more recent cars are less unreliable than those built in the earlier half of the 2000's, however don't get carried away. Renault still rank only 31st of 33 in the What Car 2011 reliability survey. Only Alfa and Land Rover fare worse. Look back to previous years surveys and it doesn't get any better.
Owning an Espace is a love hate relationship. They are gorgeous when they are working. Good to drive, extremely refined, comfortable to travel in. You trade that against the fact that you are constantly on guard for the next sign that something is going wrong, a strange noise, a judder, the mpg starting to drop, unusual smoke etc. Before long you will get that sign and don't expect many trips to the garage to cost less than £300-400. Even fairly routine repairs seem to cost several times what they would on a regular vehicle, or even rival MPV's such as Sharan or Galaxy.
As to your question about engines, I would certainly try and buy into the 2.0 dCi 175 if you can and avoid the 3.0. The second generation post 2006 3.0 dCi is fairly old technology, perhaps more so than the engine it replaced, but it does seem to have fewer inherent problems.
Buy any Espace diesel and expect it to visit the garage several times a year, and expect to spend a lot of money on the repairs when it does. If you get one that doesn't require it's own reserved parking space in the repair shop then treat it as a bonus. You buy an Espace for the love of it, not for any sensible reason.