Organizing music library with iTunes + MBP from scratch [HELP]

So I've been bedroom DJ'ing for a while now, having played at some small local parties. There is one thing that I drastically need to improve, and that is my music library organization. I started out with using regular folders, but after doing a lot of research on this community and other sites it seems that the general consensus is to:

I tried to as much research as possible by myself, since this is an often debated topic, but still have some questions left which I hope you guys can help me with.

My situation:
- I have two computers (Macbook Pro & Windows desktop) and an external HDD.
- I have a fairly big music folder with several genres (Deep house / DnB / Electro-house / Other (hip-hop, pop songs etc)
- A lot of these songs differ from tags and are sometimes incorrect

My questions:
1. What would be the best way to organize my library from scratch?
Make a new iTunes library and import everything? Or make sure to correctly edit all the ID3 tags first? Is there an efficient way to do this (scripts/programs)?

2. Should I set iTunes to copy all songs into the library on import?
I've read that it on one of the DJranking s blog that this will keep all your music in one place and is therefore very useful. I'm not sure what to believe about this, it might be very convenient to also switch between computers. Should I edit the ID3 tags first then before I import everything to this new iTunes library?

3. What is the best way to organize this with two computers?
I use my desktop for practicing at home (bigger screen / faster), and I use my MBP to perform. I usually download songs on my Windows computer. Should I use the external HDD for both computers?

I greatly appreciate any advice/help/input anyone has to share on their experiences!