ice
 

is warband any good?

I like it a lot. If you enjoyed the normal mount&blade you should get it. It adds a lot of nice options to the single player (become King yourself, find a nice lady to marry, new faction, etc.) and multiplayer is really fun too.

multiplayer is like the single with more people, or more battle based?

Multiplayer is completely battle based, with multiple modes like siege, battle (team last man standing), team/deathmatch, etc.
Usually you have 1000 Gold to buy some equipment and for kills you earn more money (only for that session) and you can buy better stuff for your next respawn - if you loose too much money by dying, you'll have to get the basic stuff again.

I would love to see some overall progression on your char, but sieges are still a lot of fun.

do I need to conquer a nation to become king myself? didnt see any option to start as it

You need to conquer a castle or a city to start it, so it's more a late game thing - and if you start with this too early, you'll make a lot of enemies :-)
But it's a big step forward, as in the original m&b you could only create a rogue empire, without assigning fiefs to your companions.