Hi everybody. I haven't played 40k since somewhere around 3rd edition and haven't followed the game in some time, but I'm getting back in after playing some demo games at a local comic store. I'm trying to narrow down where and how to start and doing some research, but I need some advice. I've narrowed down the first army I want down to Genestealer Cults, Mechanicus/Skitarii, or Blood Angels. Beyond that, I need some advice.

1. Gameplay wise, how forgiving are those three armies for a new player? I don't know the difficulty curve, though I assume any kind of marine army including Blood Angels leans towards newbie friendly. Also, just randomly curious, how popular are those three armies?

2. What is a Decurion? I see it mentioned frequently. Is that a general term for the mega-detachments that are made up of other detachments (like the Space Marine Gladius thing)? I've read the Mechanicus/Skitarii doesn't have one, is this likely to change any time soon? Are there plans to merge the two armies since they seem to belong together?

3. For Blood Angels, two things I've always loved about them are Terminators and the Death Company. Just curious how these generally perform in the current edition. I read something about ranged terminators not being that great, but I'm way out of the loop so I have no idea.

4. If I start with Genestealers is the Deathwatch Overkill game the best way to start? I'd seen it recommended and it seems like even if you didn't care about the board game it's still a good deal for the amount of models you get.

Thanks for any help, I'm just trying get up to date on things.