I multibox 5 accounts on a 2mb connection - bandwidth is not realy an issue, the main problem you will have is latency, mine is around 75-200, yours will probably be higher due to location.
your machine spec is OK - i personally would go with a better GFX card - a GTX260 or better if you are looking to run 5 at a decent fps per instance.

3 different toon types might be a bit of a headache for you first off, but you should soon get into the swing of things.. Group composition is one of the most debated topics on these forums, as each persons seperate playstyles fit better with different characters. If you find something is not working you can always swap characters early on to get the balance you fell right with.

As for the realm take your pick - depends if you want pvp or not - on the wiki is a list of whos on what realm.