Welcome to Librarium Online!
I did a Warmachine demo last night and really enjoyed it so I'll be getting the battlebox next week.
However, what do you add to it next to get to say, 15 points? I saw that it was the smallest tourney bracket where I live so I figured i'd get the army up to that level after playing with just the battlebox for a bit.
Note: I've got gift cards to get the battlebox but won't have money for more stuff for a bit so I'll probably not be able to switch out 'jacks anytime soon.
Thanks in advance!
A battle box is about 15 points
I cannot be certain about all of them as I'm a new to warmachine aswell
Each battlebox has a bit more than 15 points in it. With the bonus jack points each Warcaster provides, they're all legal 15pt lists. Building a list with just a bbox will give you a bit of room to add a small unit or two before having to move beyond the 15pt bracket.
I don't have the book on me at the moment to check exact points, but I'm fairly certain after considering Stryker's jack points you sit at a 12 point force, in which case just adding a Journeyman Warcaster could help wonders by fueling those great Warjacks you get in the box and giving Stryker that extra bit of focus to sling some spells around each turn. Other than that, you could always add Arlan Strangeways as you'll probably want him in most lists you run anyway.
World Eaters: W-47 D-10 L-9
Necrons: W-19 D-3 L-2
Protectorate of Menoth: W-36 D-0 L-7
The Battlebox (the same for most factions) actually comes up to 11 points, so you have another 4 points to fill.
At that points cost, you would seek something versatile and that does good damage (well, for Cygnar). Most people go with the Black 13th. They are a very good toolbox unit and brings the list up to 15 points as painlessly as possible. Caveat: the general opinion of B13 are that they are a bit of a pain to assemble, but they are worth the effort. Furthermore, they also add a Unit to your Warjack/Warcaster-only (aka. Mangled Metal) Battlebox list.
Failing that, you can always choose to get a couple of utility solos like the Squire, the Journeyman Warcaster or Strangewayes. However, these guys are Focus-multipliers and you would probably be better served using them only in larger games. At smaller points, you probably want more direct combat use. I would, but YMMV.