The Rhulic merc contract is called
Searforge (Not
Seaforge, as is commonly mistaken). Good place to get started is the
Battle College (linky takes you right to Searforge section).
There are two casters: Durgen Madhammer, who is very much an infantry caster (a CRAZY infantry caster), and Gorten Grundback. There are 2 light 'jacks (blaster and gunner, perhaps their best offensive 'jacks) and 2 heavies (the Driller, which is a poor man's extra slow Khador 'jack, and the Rockram Wroughthammer, which has an identity crisis, but will probably be good with the new solo 'jack marshal), 3 units (Highshields, Herne & Jonne, and the forthcoming Horgenhold Forge Guard) and several solos, the primary one being the Ogrun Bokur, which is really more of a light 'jack by proxy.
I love the little guys and field Madhammer (though often as 4-star so I can get Alexia in), and am looking forward to the Forge Guard and the new 'jack marshal (in fact, I picked up a Wroughthammer just for him!). I like fielding a heavy with them, but i know it's not optimal, and the better build is to cram as many bokurs into the list as possible (Searforge allows 3).
I kinda started backwards: I was picking up a driller and a couple of light 'jacks to use for VSF at a local store sale (40% off!) when I saw Durgen's blister and said, "what the heck". I already had some mercs for my Khador and Cygnar; this would give me a way to field a straight merc army. I wouldn't recommend starting this way, as I went into it with very little planning. If I were to do it all over again, I'd start with the Gorten boxed set, a bokur or two, and a blister of blasters. Play some 350, then wait for Thor Sternheimer and the Forge Guard to come out. 500 points gets you Gorten, 2 blasters, 2 gunners, 2 bokurs, 8 Highshields, 7 Forge Guard, and Thor, and you could probably futz with that to squeeze in a driller or two if you really feel the need for a heavy.