The box comes with Gorten. Durgen is the one you buy separately.
Anyways, there are few things to know about using Rhulic Mercenaries that will influence your future purchases.
1) Rhulic warcasters can only use Rhulic warjacks. Currently, the only ones available are the Driller, Gunner (you have these), Blaster, and Rockram. They cannot use any other mercenary warjacks.
2) Rhulic warjacks may only be used by Rhulic warcasters. If you decide another warcaster looks interesting (Fiona the Black, for example), you cannot field him/her with your Rhulic jacks. You would have to buy new ones.
3) To field all Mercenary armies, you have pick a Mercenary Contract and only use the models available to that contract.
This is a link to the
PP Forums where they have all of the contracts listed with all of the models available to each contract. The Rhulic warcasters will work for The Highborn Covenant, The Four Star Syndicate, and The Searforge Commission. If you get Boomhowler & Co., however, you will be forced to play Four Star as neither of the other two contracts can hire them. This is not necessarily a bad thing because Four Star probably has the widest selection of models available to it, but if you're looking for a theme, you won't find it here.
Actual Advice -
Gorten works well with any contract. He plays like a slowly moving pillbox, and his army choices should embrace this. The Hammerfall High Shield Gun Corps works great with him, so he can run any of the contracts fairly well. Nyss Hunters also work great with him, as his spell Solid Ground protects them from Blast Damage while they provide devastating anti-infantry shooting.
Durgen has a spell called Primed that works great with melee infantry. Since Searforge has none of that yet, you'll probably want to try one of the other contracts. Kayazy Assassins, Nyss Hunters, Steelhead Halberdiers, and Alexia Ciannor's Risen are all good choices. Durgen is a very aggressive warcaster but is much less durable than Gorten. This means he has a steeper learning curve.
Boomhowler & Co. is a decent unit, but they're kind of a Swiss Army Knife. It's tough to tell which tool is right for the job, and they're expensive (points and $$-wise). They work better with Gorten as Solid Ground prevents them from being knocked down when they pass their Tough rolls. Durgen could cast Primed on them, but they're kind of expensive to just make explode like that.
I mentioned Nyss twice, and I don't even own them.

They're offensively potent, but defensively fragile and expensive points-wise (though not as bad as Boomhowler & Co.). They're glass cannons, basically.