Good showroom appeal (my Mustang is silver with red leather) but may show up more dirt (blue denim tends to rub off onto the leather) so might be considered less practical especially in a convertible. Wouldn't put me off though...well, the SLK would but not the seats.
Depends entirely on the exterior colour. If it's silver, grey, white or black it should look OK and you won't struggle to sell it on. You might also get away with dark blue. Just don't pair it with anything silly like green or red.
Why not. 'Resale' cars make the roads an unremittingly dull place. I love wild colour schemes, particularly those that wouldn't look out of place on a '70s concept car.