is it hard, or even really possible, or expensive
I don't think it'd be hard, it looks easy(compared to a desktop) although I haven't done one.
First you buy a notebook kit,
then you pick a cpu, hard drive, optical, ram, OS, etc, and put it together.
It'd be expensive though, as a kit is like $600-800 usd, not counting everything else.
I would not suggest you do so. All laptops/notebooks require special tools for assembly, because they are very small and there's very limited space for you to work with. I've been in the laptops/notebooks industry for 4 1/2 years I've seen many customers destroyed the CPU's or mobos or altogether. Besides that, the price is also very high comparing to the comparably equipped desktop PC.
thanx for all the comments i will stick with my main idea and build a desktop