I was going to suggest coloured side decal as I really like that look, especially on white.
The reason I didn't is because the car is a cab and I assume has a black hood. I think cars look best when they are "monochrome" by which I mean two colours. Usually one is silver because of the wheels, headlights etc. and the other is the paint colour. With a cab you have a hood also and unless the car is black or Cobalt blue the hood will be contrasting (as only black or Cobalt hoods were available on the 944 cab). To be monochrome you'd need black or white wheels and a black decal, which to be honest would look rubbish.
White isn't a realistic option, but you could go with a coloured hood, though really it would have to be blue to look right in my opinion (you could get green or burgundy also), and then a matching blue decal. How that would go with your interior I don't know.
Next problem is what graphic to have down the side. You can't have "Turbo", "944" is a bit short and "Cabriolet" is best left for XR3i drivers. I quite like the Ford GT stripes currently copied as options on the Fiesta and Focus ST versions, especially in blue on a white car (not necessarily as well as the "Viper" stripes Ford use) and I meant to suggest to Rick last time I saw him that a similar stripe with "944", "944S", "944S2" etc. might look good on the 944. They would work OK with silver wheels, but I really like the matching rims and graphic look.
Leave the mirrors white - I think the contrast mirrors on the 997 GT3RS are a mistake personally.