Hey oneinam, I've heard stories about car thieves doing that trick at gyms before. I've come to the conclusion that anyone who steals is the lowest of the low and also that gyms are over rated crime hotspots. Oh, and LA Fitness are a terrible outfit! But, back on point...,
Generally, like gingejbee said, hatchbacks will usually come with folding seats and saloons won't. Mondeos, to use your example, could be either. If the boot glass opens, it's a hatchback, if not its a saloon. So a hatchback could be described as having 5 doors or 3 doors. A saloon will have 4 doors and may be called a sedan. The Mondeo comes as both a hatchback and a saloon.
I've had two 3-series saloons, neither had folding seats (though one had a ski hole where the rear arm rest was!), nor did my Audi 80 (predecessor to the A4).
I have no idea how much Merc add on for the option, but if you are really keen on the extra space, and you want to put your bike in the back, why not go for the estate version? The c-class estate is lovely!