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you say that so much prettier than i do...........

The bigger consideration is that doors are not speaker cabinets, in factory form.
And speakers require "cabinets" (or at least a "baffle") to work properly.

You want to ensure that there's basically no way that the sound off the rear of the speaker can't escape from the inside door cavity, as it will leak around and [being exactly out-of-phase] will naturally cancel the sound from the front of the speaker (most prominently in the lower frequencies).

The magnet in a speaker is a part of a magnetic circuit...
Magnetic energy flows out the back of the speaker into the back plate, up the pole piece, where it jumps "the gap" (where the voice coil is suspended) into the top plate, and back to the other pole of the magnet.
Being adjacent to other metal - even directly touching other metal - won't affect this magnetic circuit, so performance won't be affected either.

I'd address those door acoustics, though.

I am a bit worried that the magnet from the speaker will be attracted to the metal and that this could affect performance? Should I try to cover the metal part with something that will shield it from the magnet?

actually its the other way around....the metal from your door will be attracted to the magnet....it shouldnt affect anything but a sound deadener wont hurt....

I am currently installing some Alpine Type-R speakers into my Peugeot 206 and because the woofers are so deep I have had to cut away the back and some of the sides of the plastic speaker basket inside the door. The problem is that some of the surrounding metal from the door is now showing which means when I put the speaker inside, the metal is fairly close to the speaker.

I am a bit worried that the magnet from the speaker will be attracted to the metal and that this could affect performance? Should I try to cover the metal part with something that will shield it from the magnet?