Surely people arrive at a spec they will accept nothing less than and go from there.
Personally, I'd buy new if I could, because I usually spec high amounts of, or odd options, I'm unlikely to find on a used car, and also because I actually don't mind the wait to be the first owner.
I waited 8 months for my GTI.
I didn't spec RNS510 or Bluetooth as they are easily retro-fitted to the GTi...but on other models in the line up, I'd probably spec them from the factory as I couldn't be arsed ripping a new car apart to fit them...and also to get the warranty cover on certain options.
Usually, I'd keep a car 3-4 years, so spending £6000-7000 more on a new car or one with lots of factory options is such a small price to pay for having what I want every day, that i'm happy to pay it.
I know I'd not be happy with ANY car if it wasn't the 'perfect' spec...I certainly wouldn't/couldn't keep a car 3 years plus wishing I had this or that option or colour.