With my new starter sets arriving soon I have been reading the wiki. Does this game have straight opposite sides as in other games or is each army just a mix and match of any units?

You have a choice. You may play with army without faction, so you may give any unit in this army, and it doesn´t affect you... But you may take same cool faction (which give you some bonuses, like better Lowborn resistance, or better morale etc.), like France, or maybe some papal faction, and then you must carefully pick units in this army. Because, for example, in France faction, you must have at least 2 Franc Chevaleurs (or more), otherwise is this factin useless. And on the opposite side, you may have english units in your army, but they will be very unhappy in France army, so their morale will be very low... But there isn´t any "armybooks" etc., only some units in some armies have bad effects. Hope that this post will help you.

Thanks..... Next question
What is a special unit?

Depends. Some factions will see one unit as being elite, while other factions see the same unit as being special. It Really does depend, on the faction. If i remember Correctly, in a Angelic faction, demons like ymps are seen as special, where as in a Demonic faction, they are just normal troops, or elites.

And they cost will be much higher, I thing...

with two separate accounts you'll be able to trade your units back and forth and use the advantages of the leader to build a faction that has the strongest components working in your favor. As soon as you can get some spearmen and swords men they make up the basic core with the Billmen your elite foot soldiers.