Most of the people on here are from the US, we don't have the diesel models in the US, all of ours are gas (petrol) unless I'm mistaken. So it is likely that the problems you are having are not familiar to most of us.
The fuel guage could be a number of things, from bad grounding wires to a bad sending unit, or even a bad gauge cluster. changing the sending unit shouldn't be too hard. check under the back seat cushion to see if there is a panel to give access to the fuel pump/sending unit, If not then you just have to drop the tank. I'm not really sure on these cars, either way it's not very difficult. (just make sure the tank is almost empty if you have to drop it, a full tank can be heavy and all that gas sloshing about can make it hard to handle)
The passenger door is likely just something popped loose.
I'm afraid I really don't know about the other stuff, I'd suggest getting a shop manual and giving it a go.