For Vanessa, the heal is nice, but not really necessary. Its her 0 action obey that is really great if you are running a Pariah of Iron Levi list (an extra action for Lazarus, Mechanical Rider, Ashes and Dust, Joss, etc. is just awesome). Add that to her incredible shooting action and she synergizes very well with Levi.
Joss is an amazing model all around in any list, but is very useful with Levi due to his ability Creative Salvage. Its not hard to kill stuff with Joss and getting those extra Scrap Markers can allow Rusty to summon an Abomination every turn without having to use up her 0 to drop one herself.
The Mechanical Rider is an amazing objective runner + summoner from turn 2 on. A Metal Gamin every turn is very useful as they are pretty sturdy on their own for a 4 Wd model and can help out your Abominations with tar-pitting models. Plus, the reactivate on turn 5 for any of your non-leader models is pretty good.
The Hodgepodge is good for exactly as you described: cover and SS generation. Levi excels at picking a model every turn and saying "that is going to die", so that extra SS from killing a model with Levi is very reliable.
I can't speak on the Necropunks... I generally run a list with all expensive models and rely on summoning through the game to out activate, which leaves me no room when building the list to add cheaper models.