At that mileage get the petrol - it'll take far too long to make your money back...
The insurance for a one with a 19 year old would also be a LOT cheaper than it would for a cooper - something worth considering.
Same roof colour - why not just get a one for round town. You'll never notice the difference in performance in town centres. Even an auto one should be fine in town
15" wheels all have the same tyres regardless of design
Take it you mean the cloth seats? I'd upgrade to sports seats in cloth - much better design and more comfortable - cloth seats are fine in cold or hot weather
Space saver sits underneath the car, and is lowered from inside the boot. Only problem with this is the wheel off the car doesn't actually fit in the holder for the space saver. HOWEVER it is handy to have
On board computer - depends whether you want all sorts such as range, mpg, exterior temperature etc...
White indicator lenses - must have = look 100% better
Darkened rear glass - looks really good, especially on black cars
The roof lining is normally a creamy grey colour - for you I'd definitely go for it
Interior light pack - bit of a gimick really - front reading lights and mood lighting!
Smokers pack - gives you an ashtray and lighter instead of a bung thing for the socket - you don't get an extra socket
The clutch in the r56 is a lot lighter than the r50, depends what you like really! The ideal thing would be to try both, although unless you're near london this can be difficult.
Best thing to do would be to spec the pepper pack - would give you most of what you want, then add the bits it doesn't
Hope this helps.