Mark, when reading gaming materials on public transport I usually have an alternate screen to flip to quickly in case a neighbour starts reading over my shoulder. I've found that pretty much anything, up to and including hard core bukake porn, garners less scornful looks than gaming rules.
Andy - here's how Theme lists work. In normal faction armies, you just get a list of everything in the faction. You can take anything you want out of that, up to the FA for the model / unit, and agreed PC for the game. This becomes more complicated in a mercenary list, because you have to choose a 'contract' from the four options. For Bart you can either take 4* (ie the baddies) or Talion (ie the pirates). I can't remember if you can go Highborn, but don't bother, you won't want to. The contract allows you to field:
Four Star = any merc who would fight for Khador or Cryx plus a couple of other listed ones, Piper and Herne & Jonne I think, but don't quote me (shown on the card, you can check this in Battlecollege)
Talion = any merc with Privateer in its name or descriptor. Basically any jack, or any pirate model / unit.
So although Talion is the obvious choice for a famous Privateer Warcaster, you will lose access to some of the units you want.
Moving on to Themes, every caster has a theme list (some now have two, but not Bart). This will list ALL the models you can take in the theme. In addition there are four tiers, at each stage if you meet the requirements you will gain a bonus such as bonus speed / deployment / reduction in cost etc. To take the theme list you may ONLY use the models listed for the theme. The point of theme lists is that they are fluffy, themed to the caster's background and not necessarily their rules. With a few exceptions, most theme lists are more restrictive in limiting options than the benefit you get. As I say, there are exceptions, but this is generally true.
With the list of models you've posted, you'll want to go 4*, not theme list. As far as their effectiveness:
Mariner and Buccaneer are great jacks. Remember the Mariner needs a model in BTB to fire, so have something cheap and unimportant earmarked for that. Bart himself can do it, and gain Lash in the process, but that's not always useful - it also puts you in line to get AOE-ed by something targeting the lower DEF jack. The Bucc's knock-down effect is less crucial given Bart's feat, but still really good. Consider marshalling this with Sam McHorne if you bring her, as it can assault and KD a hard target, then she can pronto it out of the way before charging with her unit gaining the Trash bonus. That will wreck even heavy jacks.
Pressgangers and Sea Dogs are really the bread and butter of a pirate list. I'd say get one unit of each (or set up to convert them which might be more fun!) but if you do this, you'll also want as many of the privateer solos as you can get. Pirates are a very mutually-supporting list, everything tends to get better by being near everything else.
Sam and the Devil Dogs are great for hammering hard stuff once it's knocked down. I don't like the models, but they're very cheap for what they can do. Synergistic with Bart's feat. Quite squishy though.
Steelhead Riflemen will give you a bit more ranged ability than the pretty inaccurate Privateers. In a Bart list you'll probably be working off jacks for heavy damage, and units / solos to charge in and kill warriors in melee. These guys are in a bit of a limbo position therefore, doing something you dont' necessarily need (ie putting out a few accurate and medium-strength single shots). Get them if you like the models (which I do) and remember if you expand into other 4* casters a steelhead-heavy list works very well.
Croe's. I've just bought the unit, played it twice and had difficulty doing much. Lowish MAT and low RAT with no CRA/CMA means they lack a bit of punch for their price. Having said that, against certain targets, especially low-DEF warbeasts or heavy infantry, poisoned attacks are nasty. Backstrike is incredibly hard to pull off without a Telekinesis spell, but if you do it it's very funny rolling 5d6 on a charge damage roll!
Solos - if you can, just get all the privateer solos and be done with it. If you can only get a couple, Dougal, Rockbottom should be first, then Hawk for offense, or Bosun Grogspar and Doc Killingsworth for defence. Dirty Meg is nice but only really worth it if you want to go jack heavy, as she can either marshall a melee jack (leaving Bart to focus on ranged) or just walk around repairing stuff.
Kell and Eiryss are amazing solos. I love them both and can't recommend them enough, even though they're not exactly pirat-ey
Bokur is situationally useful. Personally I'd leave him out of this list as there are better things to include. A good utility piece to have though.
So order of purchases - I'd probably say get Mariner; Dougal; Min Pressgangers; Max Sea Dogs; Hawk, Grogspar, Rockbottom, Doc; another jack, maybe Mariner, maybe Mule (he can take non-Privateer jacks as well).
After that, it's up to you whether you want to take different units like SH Rifles, Croe's or Devil Dogs, or to max the PGs and take another unit of them or Sea Dogs. You can also think about the SD UA Mr Walls, and Riflemen to bulk out the unit to the full-on Piratestar
