[Shadow Stranger]

"1. What is most important for Wizards and Socerers to be most effective ie. intelligence and/or wisdom etc."
For Wizards it is Intelligence, for Sorcerors it is Charisma, it is quite one-variable.
"2. are there restrictions on certain classes multiclassing ie. was thinking about the following as want another fighter to use a crossbow"
Apart from alignment and armor considerations, no. Certain classes have alignment restrictions. Rogues get penalties to their skills for wearing heavy armor.
"3. also if a sorcerer m/classes into a fighter can he start wearing armour (would that affect any spells being cast) and use an axe, sword, crossbow etc."
Unlike Second Edition, this is a matter of feats not classes per se. Magic users can choose armor-use feats but they would do well to choose spell-boosting feats like sub-vocal casting.
"I just don't quite understand the whole multiclassing thing. I definitely want to have a crack at it but it seems like cheating a bit as you get 2 for 1."
Unlike 2E there are no freebies. You don't get 2 for 1, you get a lower level in your Rogue class if you take a level of Fighter. Though it would be still a good idea. I have a Rogue/Ranger who alternates between taking a Rogue level and taking a Ranger level. Generally for spellcasters, until you hit level 20 you only want to take one level outside of your spellcasting class.

Read the IWD2 manual, that will give you an idea as to basic aspects of the Third Edition.