The only things you'll want for FEC that you can't get from multiple start collecting boxes are:
* Abhorrent Archregent. Not too pricey as a stand alone hero, but also comes in the broken realms box which should still be available most places and is a pretty decent deal in its own right
* Varghulf. Expensive and annoyingly in resin, decent details but very stiff pose. If you play at independent stores where 3rd party models are allowed, I'd look online for non-GW alternatives. If you are stuck playing at GW stores or wish to participate in official events, try searching online for conversion ideas based on the crypt flayer/horror kit.
* horror/flayer courtiers: no actual models, so you're going to end up dedicating a whole set of three to converting some of these.
* ghoul courtiers: no actual models, and not too commonly used, so you may just opt to skip them. Alternatively, the Warhammer Underworlds ghoul warband has a handful of cool ghoul models you can run as courtiers without having to burn an entire batch of 10 ghouls for just a couple heroes.