Hi Alasdair.
Well here's a dilemna for you, go for it now and get someone else's spec or wait and get YOUR car in March. There's several things to think about if you are interested in it's resale value, firstly when you are likely to change it. I hate to be the one to say it, but it will lose about £20,000 in the first 2 years and that's the cost for the pleasure of speccing your own car. For resale, either silver or dark colours (black or blue) are the best because more people want those. Time of year could also come into play - values are lower in the winter but pick up in the spring so the timing of your one on order could be good.
I don't know what Atlas grey looks like but as for the other options:
Carerra S wheels - good, they're in keeping with the model
Heated seats - unnecessary unless you suffer from farmers and park outside at night
Sunroof - nice but not necessary
Bose - good
Multi-function wheel - good
Wheel centres - cheaper to retro-fit
Rear wipe - good
Others - I don't know if Litronic lights are still an extra but IMO a must. PCM (sat-nav) a must.
One other point - the money you are spending would buy a good, low mileage GT3 or even a turbo and someone else has taken the hit on depreciation. It just depends on how much you want that new car.
Whatever you do, have fun, they're fantastic cars.
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