I agree about the sun visors, Piper ones will be expensive. However the stick on sheets of darkened plastic can be purchased from Mothercare or similar. They don't have any glue and can be taken off and repositioned easily, even on the side windows.
Re the seats, its unlikely that you can fit anything other than the standard ones. You can get the std ones re upholstered. Leather works out cheapest as any fabric has to be special so that it doesn't burn or produce toxic fumes.
ZA