First time playing NWN2.
What's the best build to enjoy the story without too much powergaming?

What do you like to play? Seriously, you can play pretty much any build so go with whatever you like best.

Something that gets me the most out of Dialogue? But is it better have more combat oriented skills so I can survive until MoTB expansion? And less dipping into other classes the better.

You'll want either a character that has the social skills as class skills for the main class or one that has at least one class level in a class with them. I'd suggest looking at Rogue, which has them as class skills as well as getting a lot of skill points per level. Take Fighter for combat skills and feats and you should be good to go. Traditionally, the Rogue is DEX-based, but I often (in fact most of the time) go STR-based.

I played through with a Ranger as the main class, with one level of Cleric, one of FTR, and one of Rogue. This got me access to Rogue skills as class skills (so my maximum rank in them was character level +3), FTR proficiencies and an extra feat, and some free cheese from the Cleric domains. Worked well for me, but it's a total cheese/powergaming build.

You should take something with good conversation skills (diplomacy as class skill, its the most importand skill unless you want to be evil, then you might think of bluff) and high charisma.

I have made this http://nwn2db.com/build/?163388 as a role playing build.
But it can fight good enough. Make a human and take diplomacy instead of spot.
It is the best "holy knight" I can think of without being a power gamer.

The first thing I thought of was a bard.
After some search I found this http://nwn2db.com/build/?32688
A bard must not be lawful, but most games I end up beeing lawful good.

Warning: Both favoured soul and bard are spontanous casters. They learn spells only at level up. Take care what spells you choose.

It is possible to play a single class but if you add other classes it gets better usually.