Artificer is a great class, but only if your game is going to have enough downtime and a base of operations for creating magic items (and access to some of the Eberron specific feats to really enhance your crafting abilities). Otherwise its not really worth it. I think human is a better choice for it, but halfling might work as well if you can use something like an iron defender as a mount.
I too like the Warlock/Paladin idea, but it looks like your group really needs a rogue-type. Hmmm...can you use one of the feats that lets you multiclass freely as a monk? Try Monk/Rogue as the ultimate infiltrator. The downside there is that you wind up neededing great stats for everything. Imagine a secret order that arose in a land ruled by an oppressive warlord. There they worked to protect the poor, steal from the corrupt rich, and eventually restore the proper ruler to power. Now they travel the world seeking to end tyranny and oppression and restore justice and order.