Mass-effect – In Mass Effect, where is the small station that sells special items

If you haven't changed the default controls, q will allow you to disembark (on the PC) while the (B) button on the XBox-360 handles disembarking on the 360 version.

As Fabian mentions, you need to exit the vehicle to interact with these objects.

With Metgos in particular, be very careful when outside the vehicle! The planet is classified as a level 2 Heat Hazard, which means as soon as you exit the vehicle, heat damage begins to accrue. When the heat damage bar becomes completely full, your squad will rapidly take damage until you die or get out of the heat (by re-entering the Mako or a building).

Mass Effect 1's "Bring Down the Sky" has an effect in Mass Effect 2 and 3. The other available DLC, "Pinnacle Station", does not.

You've asked to avoid spoilers, so I'll simply say that the default canon is that Bring Down the Sky ends with the Renegade ending. If you play it personally, you can alter this.