You dont need to 5 box to see the old content. A level 80 tank in t9 badge gear can solo everything even some of the current level 80 dungeon heroics. There are a ton of YouTube videos of almost every class soloing BC raid content.

Personally I would recommend using one of the more classic and tested groups if you have not multi-boxed before. A tank and 4 shaman are arguably the easiest to get up and running. If all you want to do is old content, even 5 shaman will show you everything. You will only have to setup one set of macros and control and copy it to the rest. Its very easy to just one button face roll everything from 1-80 these days. However if you are thinking of 5 boxing for the long haul, then there are a ton of extra factors to consider. Such as group makeup, item and armor usage so nothing that drops goes to waste. Rezing and trade skill setups. PVP. etc. There are a hundred party setup threads here. Search and see what has been recommended before.