Hi Manchu, I was playing with Chaos Space Marines myself. Just some notes, because you mentioned you're waiting for your book. People have been talking about this game, but there's some important nuances that haven't been getting much traction.
1. Mark of Slaanesh isn't bad; +1 Initiative is useful
I think the Mark of Slaanesh is a little overlooked, is all.
Initiative has more uses than just breaking ties in combat; in fact, it's a pretty good "utility" skill. Spots hidden fighters, good for spotting enemies in the mission when you have to be the sentry.
The thing that really comes up often is early recovering from Pinning. Your opponent shoots you in his turn, hits, but does not wound because you're Toughness 4, you still have to lie face up because you are Pinned. Being unPinned normally happens at the END of YOUR turn, so basically he just wasted the time lying around.
However, if you are within 2" of a friendly model, you can try to escape Pinning at the START of YOUR turn, by making an Initiative check (roll equal or under your I). Now you can move and shoot or charge normally, instead of having to sit the turn out.
Full rules on page 35 of the printed book, the last paragraphs, for when you get your copy.
Recommendation - for fluff reasons, don't feel bad for taking Mark of Slaanesh just because it might not seem as immediately useful as +1A, +1T, etc. Initiative does come up a lot.
2. Your standard gear might be better than you realise
There's a lot of overkill in the lists I've been reading (not just you, a lot of new players, with or without the book) because I reckon we're still in the full 40k large-scale battle mindset.
Not sure you know this because it's rarely talked about, but did you know your Chaos Space Marine just stabbing someone with your free knife already has a -1 save modifier on him? Because they're super-strong by normal human standards?
In close combat, sheer Strength can have saving throw modifiers. Strength 4 gives a -1, Strength 5 gives -2, etc. Page 42 when you get your book.
You can probably get away with a regular Sword; it has Parry (a great ability) and you already get a -1. My experiences in close combat is, it's very all or nothing; if you're winning combat you're probably scoring a lot of wounds and something usually goes through the armour.
Meanwhile, you probably don't need the meltagun or the Inferno Bolts. Instead, ironically, a Flamer (seen as useless in 40k) is pretty helpful because it ignores cover and pins groups of enemies for you to divide & conquer via close combat.
If there is 1 piece of expensive gear I'd recommend, it'd be the Heavy Bolter for this game. Multiple shot weapons are very useful considering how many cover save modifiers there are. I was willing to pay 200+ points for even an Ork gunner because it was so vital to have a guy providing covering fire.
Recommendation - it's NOT just Boys Before Toys, but that even basic weapons get the job done. A Tzeentch Gunner with a Flamer might be fluffy an effective, actually...and also kinda ironic sounding. Also, a Heavy Bolter, though pricey, is very valuable.
3. Your basic gear might also be worse than you realise
Bolters don't rapid fire in Shadow War. That makes things like Inferno Bolts on a single-fire weapon really a huge premium.
Sadly, Eldar and even Ork basic rifles have "Sustained Fire" i.e. multiple shots.
4. Camo Gear is a meaningful and very cheap upgrade
Camo Gear in Shadow War isn't about cover saves. It's about reducing the range of enemy weapons by 4". It doesn't sound like a lot, but in a game where many units' movement is 4", it's really worth the 5 points you spend.
This is especially true for your close combat units. CSM aren't fast like Eldar Harlequins, something so cheap that lets them get that extra bit of movement before getting shot at is worth it imho. I think it's almost an oversight that it's even on the list.
Recommendation - if you find yourself with a few points leftover, please consider getting Camo Gear, maybe just for your close combat models, like your Leader and the Nurgle or Khorne Marine.
Just presenting options more than opinions, because there's some really interesting nuances to this faction. Also strongly agree with the downline cost of adding recruits; list-growing is part of list-building and I've learned to plan my Recruit/Rearm options ahead of time.