So i have returned to 40K for the first time since 5th edition. I've bought the Chaos Space Marines codex, and it's quite different. So here are some questions:-
Its actually not as different as you think, yes a fair few things were tweaked but the vast majority of elements remain the same.
1. Marks of Chaos - free but they don't seem to bring any benefits now other than the right to buy an Icon. Is that right?
One of the additions in 8th edition is catagories/faction keywords. Some rules only affect units with certain keywords. For example, Guilliman in the space marine codex has rules that allow any unit with the Imperium keyword to reroll 1's to hit, but if they have the space marine (I think its actually adeptus astartes) keyword then they get to reroll all fails to hit.
So the marks are free because they add a keyword to a unit taking them, which potentially limits what units can benefit from special rules. (The daemon prince is a good example of this, if he takes a mark then his prince of chaos ability only effects daemons with the same chaos allegiance in addition to chaos marine units with the same legion key word. So an Emperors Children Slaanesh daemon prince does not provide rerolls to bloodletters or Death Guard chaos marines.)
2. Legions - Plague, Noise, Rubric Marines and Berserkers are all in the Elites section. If I'm doing a particular legion - say Word Bearers - can I choose these guys and give them the 'Word Bearers' Legion keyword? I assume they are supposed to be a unit of troops dedicated to a particular god but within the main legion.
Yes, when you choose a detachment you choose a legion for that detachment. All units taken in that detachment have the same legion keyword. Its entirely possible to field multiple legions in one list, but that requires multiple detachments.
Also, I do believe in the latest chapter approved when you take certain legions, like World Eaters, the cult troops go from elite to troop choices and the other cult troops become unavailable. (Since the Death Guard and Thousand Sons have their own codecies they do not have legion options in the regular chaos marine codex.)
3. Obliterators don't have a close combat weapon any more. Is that right?
Obliterators are no longer armed with powerfists, correct. They can still fight in close combat but shooting is where they want to be. (In this case each attack an obliterator makes has no ap and only deals one damage per failed save. as opposed to, say, their fleshmetal guns that can have an ap and damage of up to 3.)
4. There's quite a few new troop types. Before I go out and spend vast sums on them which ones are auto-include (or close to) out of Warp Talons, Heldrake, Mauler Fiend, Forge Fiend and Mutilators.
Its all about the play style you want and the army you plan on building. Otherwise this is a fairly subjective question. Heldrakes, being flyers, are going to be harder to hit and warp talons have an ability that prevents enemies from firing overwatch at them on the turn they arrive from deepstrike for example.
5. Finally I get the impression that it's mostly about shooting now rather than getting stuck into HtH. If so should I be going for a gunline or is Chaos better off relying on Deep Striking and charging?
Oh no, no it is not. This was a fear people had prior to 8th edition coming out, I think it was also a fear for 7th edition as well, but no close combat is still very much viable. Certain armies, like guard, would rather stay out of close combat but the armies that prefer it to shooting are not really at a disadvantage. (Especially when you keep in mind that it is entirely possible to get turn one charges and charge after deep striking in.)
I've seen more than my fair share of games over by turn 2 in this edition, both from overwhelming firepower and close combat.
My old guys are mostly Alpha Legion types so at least to start with I'll probably go that way.
The Alpha Legion keyword is pretty good for both gun heavy and close combat armies. And with their wide selection of units you'll definitely be able to get some good use out of it.