Ben, TBH the best buying Guide I have ever come across is AO. OK we are all a bit biased, but you learn a lot from people posting problems. In reality their are no major issues with the 156, that you wouldn't expect with anything else. Minor or occasional glitches like Suspension Bushes, Dicky Imobilisers, and the odd Cam belt failure, are mentioned in posts. These sort of things rarely come up in Buyers guides.
I don't have much of a problem with Supermarkets, at three year old, you are out of Manufacturer or dealer warrenty anyway. It would be worth paying that bit more to get a decent 'all component Warrenty' but I am not sure it matters if it's bought from an AR dealer or a supermarket.