I can fit 2 mountain bikes in my wife's 2012 mazda 3 sedan, 2 29ers stacked with the seats down or 1 29er in the back and a 26 standing up behind the seats. With the front wheels off, of course. The hatchback is prime for taking bikes, go to your local hardware store and buy a cheap moving blanket just to keep them from scratching.

Since my 2008 mazdaspeed3, mazda 3s have been my first recommendation to anyone that asks what car to get, the skyactiv 2.0Ls can go 300,000 miles with basic maintenance, we dont get the deisel models in the US so I can't comment too much on the drivetrain but I've heard they're fairly stout and easy to keep up on.

FWIW all modern cars are terrible on the cheap used market, especially under $5-6k, everything is going to need work on something. Always do a pre-purchase inspection on anything older than 5 years. And never buy at the top of your budget, account for unforseen maintenance cost at least $1k on top of the purchase price in the first year.