Morning! Let's see if I can help out a bit and answer a few of those questions. It goes without saying, these are all my own personal experiences, and as such are super biased towards both hilarious things happening and playing against the full range of 40k armies (as opposed to other legions). Feel free to disagree or debate as needed.
1.) Since we're legion, we get all sorts of fun price cuts when we start taking larger squads. Larger squads are also a fire magnet, so its a good idea to protect them with extra armor. I've found an armored cavalry approach works quite well. Land raiders of all varieties go miles to making your World Eaters more effective, and since they're assault vehicles, you can get stuck in where you need to. This will slant your army towards the small side, but as long as each of your units has a very distinct purpose, you stand a far better chance of success. So far, my favorite list contains two Spartan land raiders and a fire raptor.
2.) For Rites of War, I've tried a few just to see how they fair. Like many, I started with Pride of the Legion (veterans/terminators as troops) so I could begin playing even with a small force. This is hella fun. Try it.
For our own rite of war, it actually combos *really* well for our more traitorous side. Having Rage *and* Hatred is a very potent combination, and make even small squads of World Eaters a pretty big threat on the charge. That's 4 attacks with re-rolls to hit from your basic legionary with a CCW. You did give them the CCW upgrade didn't you? The only reason I haven't used this more often is that my World Eaters are supposed to be the good guys, and I *really* like librarians, which this rite of war restricts. Of course, you don't really need psychic powers when your World Eaters are already re-rolling to hit
The only downside I have seen to this is them actually being a little *too* effective, especially for more elite troops with heavy duty attacks like terminators or veterans. Just like in vanilla 40k, if you wipe a squad off the face of the board with a super effective charge, you feel really good about yourself. But then your squad is forced to sit there for a turn and eat every single ranged weapon your opponent has. Makes me really miss playing fantasy occasionally when you can't overrun and contact another enemy unit to start a new combat.
3.) I will admit the only reason I ever take a praetor is when I want to use a rite of war. They aren't super expensive, but they don't really specialize in anything besides general combat (see point 1 above). My best use of them has been to keep them cheap as possible. Iron halo, paragon blade, digital weapons, melta bombs, then add him into a squad that isn't quite full strength. They can provide ablative wounds so he can can-opener terminators. Just be careful of getting him stuck into a challenge that he doesn't want. Often, Praetors are the meanest thing on the table until you hit a Primarch or a nasty CC special character like Sigismund or Sevatar. All 40k tends to favor a "girls over pearls" approach, as in its better to have more boots on the ground than a ton of super expensive units. If you're taking one, use him well.
4.) I *love* centurions. Much cheaper and much more specialized than Praetors. You should *always* buy the upgrade for them to turn them into a specialized consul. Lemme break them down:
Chaplain - While badass, unfortunately not quite as effective for us when we are using the Rite of War. If you're using another though, they are a *huge* force multiplier. While less flexible than a librarian, chaplains give your squad hatred and fearless without a psychic test, making sure they hit hard in the first round of combat and won't get swept after a bad combat or afeared by something scary in an era before "And They Shall Know No Fear."
Master of Signal - Another large force multiplier. Gives their squad the benefits of Night Vision and an Augury Scanner. Keep them super cheap, and hit something juicy with their first turn bombardment. After that, as long as you don't shoot (one boltgun is typically far less valuable) they can give any unit that isn't a super heavy or an independent character +1BS. Very good if you've got some heavy support squads or *really* want to hit with a *ton* of Fury of the Legion shots.
Legion Champion - A cool model and a neat idea, but unfortunately suffers from kinda being a "Praetor lite." He won't do anything your praetor can't do better, and doesn't have any good specializations like some of the other Consuls. The last thing we need is another combat beatstick
Vigilator - Very valuable for their Scout rule. They can get a squad close (which is never a bad thing for a World Eater) just make sure you aren't putting them in a position where your opponent will have no other threats to shoot at, and thus just focus fire the entire scouting squad off the board in the first turn.
Librarian - One of the more expensive options, but commonly worth the price of admission. Even with the wonky new psychic phase, they are still a big force multiplier. Bonus that legion librarians can get any of the five disciplines and are better shots than their Sorceror/Vanilla marine counterparts. No longer a negative that they have a slightly lower leadership thanks to 7th ed
If you're going to be spending the points on these, it's typically a good idea to go big or go home. A single lvl 1 psyker tends to get bullied around by a psyker heavy army like eldar. A couple of mid to high level librarians do very well to even out a few problems in your lists, support each other well, and can force at least a couple of useful powers through every turn. I prefer biomancy myself, but play around with a few disciplines and see what you like.
Just don't be an invisibility douche bag.
Forgelord - Another good force multiplier. Can sit in your land raider and repair hull points, or can go in a squad to provide delish rad grenade goodness.
Siegebreaker - Unfortunately not quite as good a buy. Typically wrecker and tank hunter are less valuable than +1BS outside of a few fun specialized scenarios.
Primus Medicae - Typically seen when players are crafting a large, expensive command unit. I avoid using the term "death star" because I'm kinda sick of hearing it (everything's a something star when it can't be shot to death. Beaststar, screamerstar, seerstar ) and it implies there is an inherent weakness.
But! Great for getting a 5+ Feel No Pain save into a squad that typically couldn't have one. Up to you to decide whether or not the price of admission is worth it. They typically end up the price of 2-3 normal apothecaries. It is *hella* fun to have a Cataphractii armored medic though.
Medicine through the brutal application of chainfist
Moritat - I know the dual plasma pistol chainfire thing is still alive and well, but I really don't think it's worth it in most cases. Any smart opponent will just gun him down if he's on his own. If you take a support squad of destroyers, he and his retinue typically cost more than whatever you're going to obliterate.
I will say it was fun to plasma pistol a Knight's rear armor to death with a jetpack moritat though. Even if he did kill himself with the "Gets Hot" rolls afterwards
Mk, that's a little more than halfway done. Decent start?
Let me know what else you'd like included, and I'll get back on answering those and any new questions shortly.