L2 client, and steam version

[Denny]

Greetings
I'd like to ask for information on the following matter. I have Lineage 2 installed on my PC, and I wish to purchase the game. I'm inclined to purchase it via Steam, though there's no real price advantage. It's just Steam
Now, the questions, that arise are:

1) will I have to download the client via Steam, or can I copy/paste my current client into the appropriate folder? Will Steam adjust whatever files it needs to (if at all)
2) Once that's done, and dealt with, does Steam force itself open for the game to run, or can the game then be launched without Steam?
3) I have an NCSoft master account, and a trial. Will I be able to use my current account (Both master, and L2 specific) with the Steam purchase?

The first 2 are also related to my infinitely infuriatingly low (15GB/mo) bandwidth. I know Steam is a BW ♥♥♥♥♥♥♥, so I'd rather not launch Steam when not necessary.

Thanks for any, and all replies.