New theme or customize

Hi

I wanted to ask some advice regarding the easiest, fastest and most flexible way to create a custom theme.
I am willing to learn to create a theme from scratch from a .psd file, but never tried it. I have done a decent amount of customizing the twenty ten theme on my wifes fashion blog .

I am hoping to make a theme where the reader get the feel that they are in a cockpit of a plane, I haven’t made the design yet so don’t know how I’ll pull it off yet.

I guess my question is, how would an experienced developer do this, and how long time should I guestimate each option takes? Further I’d love to hear about some limitations I will experience if I decide on customizing rather than building?

Thanks in advance
Claus