Some people do mess with enemy
HP in 4e. It depends on the desired style of gaming, really. other groups find the opposite is true, and the enemies are too weak.
D&D4e, in my experience, works best with 'interesting' encounters. A stand-up fight with a half-dozen orcs isn't interesting, it's just a minor tactical challenge. A fight with orcs, while the party is trying to get through or around a locked vault door, and the Orcs have occasional leader-types that may summon more orcs, that's interesting.
Bonus points if it's all done over lava.
A good suggestion would be to start with some of the pre-made adventures. I don't know the best ones.
Also, 4e really does work best starting no more than 3rd level, less for new players. Higher level characters tend to have a lot of things to remember (powers, feats with conditional bonuses, if/then abilities that trigger on certain events) and gamers can get 'analysis paralysis.' Start at 1st level, and most characters will have a couple at-wills to use as needed and a couple encounter/daily powers, and not much else to cause problems.
You cans till do a lot of storytelling inside the framework of a pre-generated module. Besides winging it if things go off the rails (they will) there's providing hooks to get the player characters into the dungeon and making characters memorable.
When to bend the rules is a big part of Gming, and not something that is agreed on. In general, let the dice fall where they fall, unless you screwed up ("Really, I thought they could take on all those dragons...") but feel free to bend the rules a bit. if the players enjoy fighting a module's boss, make sure his death follows the cinematic rule of not leaving a body... You can bring him back later, then. Maybe plus a level or two.
Don't sweat the details on your side, either. The players each have 1 character to worry about. You have countless
NPCs. If an
NPC isn't 100% legal byt he rules, it's no big deal. Especially if you follow my last piece of advice:
Let the players be awesome.
They may not be the most powerful beings around, but they are the most interesting. The story revolves around them. If it doesn't involve them, it's probably boring. A bad example of this is I've seen
GMs actually run combats between
NPCs. The players had no way to get involved, so it was just watching two
NPCs punching each other.
GM rolled hit/damage for each liek it was a 'real' combat... but we didn't care. If it doesn't involve the
PCs, feel free to just make it up and get back to the
PCs doing something.