Anyone able to help a new player out?

I've been reading the Hors Heresy novels for awhile now and decided I really want my own army to play with but have got some questions before I start building my force. If you have the time I would really appreciate it if you can answer these questions to help me get a better understanding of the game and how things work.

I have already decided that I want to play both Chaos Space Marines and Necrons. Chaos has always been my favorite and I love them purely for the models. I plan on making a fluffy list just for casual play, using whatever models look cool (not decided on a Legion yet) and will probably play Necrons as a more "serious" force with more strategy.


A) I understand that there are different types of playing. You can run an unbound army where you are free to take whatever you want. Then there are formations or detachments and you have to have certain units and you get special bonus rules and buffs. I think Necrons have their own special kind of detachment called a decurion but I'm not 100% sure.
How important are these detachments/formations? There's 2 things I'm afraid of. 1) I don't use a formation and my army really suffers from not gaining the special bonuses. or 2) I buy a bunch of models for a formation, then the next codex comes out and I have to buy all new stuff because the old formation is useless. I like the idea of picking what I like and I also like the idea of getting buffs. I don't want to shoot myself in the foot when building my first list,especially because on different forums I see that (apparently) Chaos is in the lower tier of playable armies.


B) Okay, suppose I want to paint my CSM army using the Thousand Sons colors. Are you allowed to take a special character like Kharn, paint him in Thousand Sons colors, and use him in your army? Now, I can understand if you are not allowed to put X character in a Y army. What I mean is, "Hey I know my army is painted like Death Guard, but they all have marks of Slaanesh and I have Fabius Bile as my HQ painted in Death Guard colors."


C) Can you buy Chapter/Warband icons? Like if I were playing Blood Angels and wanted their symbol on my armor and banners or maybe on tanks. Do they sell stickers or stencils for this?


D) There are a ton of different weapon options for your models it is a little intimidating for a new player because I don't know what is "good" or not for how I want to play. Is magnetizing weapons easy for most models, including HQ's?


I think those are the major questions I had. I am thinking of starting with Necrons first. Their detachments seem a little simpler and with Chaos there are just so many options it's a bit overwhelming. From what I hear the Necron detachments are good(although this was told to me in 7th ed) and maybe following a detachment outline might make army building easier for a new player because there's already a rough guideline to follow rather than, okay just go buy stuff!

I know I need to pick up the Necron (and Chaos)codex but are there any other sources you would feel that would help me plan out my force? Ive tried reading army lists online but it's hard when you don't really understand how good those weapons are or their matchups I should say.

I am really, really excited to get started playing. I currently play Warmchine and Hordes with some friends but we were talking about starting 40K (some of them also play Age of Sigmar but it never interested me) Work has me running long 12+ hour shifts so I'll try to check back on here when I have time.