(in dread, do the hearts get harder higher levels?)
Nightmare has only slightly more hp than Dread, but Lethal has twice as much hp as Nightmare. That's not the worst of it though. The real ballbuster is that you don't get to magically kill all the zombies in the area by finishing off the heart (aside from the one feral that always seems to show up late to the party). Not only are they all still alive, but there's a shitload more of them still incoming. And every special zombie on lethal is the plague variant too.
ScentBlock still trivializes it though. I run in, drop a C4 next to the heart, burn one mag from a MPX, detonate C4 and reload, then half of the second mag and it's dead. Quickly loot the plague samples if possible, otherwise GTFO and come back later if I really need anything from hearts still. I can usually get back to my car without being attacked and depending on how close other hearts are you can sometimes get 3 or even 4 of them down before the ScentBlock wears off.
I'm still trying to figure out which survivors (or perhaps more accurately...skills) I need to bring when I start.
Assuming you don't have Red Talon dudes, I'd go with Lichenology, Pathology, and Pharmacology (used to do Munitions first, but that skill is moot now that there's a free Demolitions guy)
It all depends on how many people, which skills, and what kind of base you intend to have in the long run though. Like the farm setup I listed above, that's with a crew of Hacking, Pathology, Lichenology, Plumbing, Pharmacology, Demolitions, Automechanics, and a couple others that are just kind of dead weight. One doesn't even have a 5th skill iirc, since I never had a skill book trader show up on this map. I used to always use an Agriculture guy, but now I get all my food production from outposts and only need the Lichenology guy to counter the Infirmary upkeep and provide Gardening/Herbalism when I decide to use Hydroponics.