Go kick the tires of all of them and buy the one with the less sketchy looking seller.

The Mazda 6 is the boring option, but it will probably be the cheapest to maintain in the long run, especially with the low miles. With the BMWs they are going to rip you off every time any repair is needed. Might be similar with the Volvo, but the S60 is cool. I have never dealt with diesel, so probably best to ignore my opinion, but I wouldn't worry about it, they tend to be solid.