Multiboxing advices

Hello guys I'm new to the multiboxing thing and I'm so hyped to try it out on Classic.
First I'd like to ask if there's any private server where I can experiment and get things in place before Classic comes out.
Then I would ask your opinion about what class is the easiest to start, I want to farm dungeons (trash) and so gold, I want to try what I never achieved in WoW and that is to be among the richest players in my realm. I already enjoyed every aspect of the game but the economy is what is left for me to explore.
I will play alliance and I can't decide between Mage and Warlock. Once 60 I want to farm dungeons as much as I can, what would be the best choice in your opinion? I will use 3 accounts.
Thanks everyone.

Some important information in this post. Hope that helps.