Carbon black is nicer than ruby back IMO. I find it strange that people would buy looks purely over function, in my opinion both are important. I wanted red leather seats but got black because you have to make sacrifices. Youl probably find there wasn't a 3.0 in ruby black with your exact specifications. Some things you are looking for are hard to find like the folding mirrors I understand why you want it tho, I couldn't find one with it! It would be nice but you can always just push in the side that faces the road. The storage pack can be added later, tbh its pretty useless. Heated seats in a coupe? If it was a roadster I would say fair enough but not a necessity on the coupe (but I would say a lot came with it, so youl most likely find one). But I would say the other spec will be easier to find on more examples, maybe not the two tone leather as much as the red leather.
With mine I was in your situation, I wanted an M coupe in:
carbon black
red leather seats,
Bluetooth + Telematics
10 speaker system with DSP
Xenons (come as standard on the M)
Navigation system - Professional
PDC
Cruise Control
6 CD changer
Carbon Leather interior
Dimming interior and exterior mirrors
Folding mirrors
Managed to get all of it apart from the colour and seat colour, cruise and folding mirrors. My car doesn't have heated seats either and dont regret that decision either.
I got a silver grey car with black leather which also had tinted windows, carbon fibre rear spoiler and the M wheels in gunmetal and the car looks awesome IMO. I didn't look back on the aesthetics although what I wanted would have looked better the Z4C is a car that looks good in most of its colour variants. Cruise can be retrofitted and the folding mirrors... well I live on a tight street so it would have been nice but I don't mind pushing up the mirror when parking on the street, but I have a garage so its not too much of a problem.
Few pics of it here:
http://www.z4-forum.com/forum/viewtopic.php?f=9&t=33085
I would wait, IMO finding a good car that has been looked after well mechanically and lower milage is more important. You will find people that got the car of their dreams, will have put a lot of milage on it. Thats what I found when looking.