Yup some sort of broadcasting software:

HKN is free
Keyclone has a one time fee of 20 USD
ISboxer has a yearly subscription fee of 36 USD

ISboxer has things like FTL and such built in, and also has many mouse broadcasting options that are pretty much required for boxing melees.

Keyclone is ridiculously simple and easy to use - at least in my opinion, but lacks many of the bells and whistles of ISboxer

I have never used HKN but have tried isboxer and use keyclone. Either is definitely worth spending money on, and will make your life infinitely simpler. Jamba is also the most amazing boxing addon ever written, and you will want it.

I'm sure others wil give their opinions on the various boxing options. There are also many threads already comparing them - take a look through the search function.

You will need BC and WOTLK for all accounts if you plan on playing to level 80. Vanilla will allow you play to 60, BC to 70, and WOTLK to 80.

You migh also consider just taking the plunge and buying the fifth account. Having your own group is extremely nice.