The only bits of the kit that i didn't use were the side frames - because i used a different motor block; the buffer beams - because i wanted a different style; the main part of the air tanks - easier to use brass tube rather than glue and fill the four peices of white metal and finally the door handles.
Everything else was an addition.
If i hadn't bought the kit I would have had to work out the sizes of everything. It also includes quite a few parts that aren't sold seperately (i know, i asked!). So overall the kit was worth getting as i used most of it. No idea how many man-hours went into it, i expect you could build it in a couple of weeks if you weren't such a perfectionist as myself. I still have to fit out the cabs...