I think I own everything that the Viks can take (so far released) and there are definitely some that get used a lot more than others. So a few notes of note for beyond the box:
Ronin - at least one gets taken every game, normally two. These, absolutely, should be your first purchase as they can let you bring a Vik back.
Gunslinger - often taken, less so with the Friekorps now available as I try them out (it's generally a choice between him or the second Ronin).
Hamelin - a regular until the Friekorps arrived, mostly due to obey and hungry rats (damn Ressers ), but a general toolbox. I still like him as he's been my MVP so often, but I'm trying Von Schill now and can't afford both in 35SS.
Misaki - excellent damage dealer and a great secondary (tertiary?) melee threat beyond the Viks. It's a call between her and the control abilities of Taelor, so try out Taelor a little more first I'd suggest.
Of the Friekorps the Librarian is great, she really helps the Viks out and Von Schill himself helps both the Librarian and forces your opponent to deal with a ranged threat from the crew (especially if you take the Gunslinger) as well as the more standard melee.
The Desperate Mercs will probably make it easier taking Killjoy, if that's your cup of tea, but I've always found him to be a little too expensive for a one shot missile outside of Ressers (who can summon back more than he's worth from his corpse counters once he dies ).
So... (yay a summary), if you're flush with cash I'd start with the Ronin, Gunslinger and Friekorps box set. If you don't have that much to spare, and don't intend on switching to a new Master (Von Schill) just yet, then start with the Ronin and work out from there .