It seems rumors were salty after all. There are more and more sources claiming we will have Codex next year, probably January, which is basically in line with previous leaks. As this is quite some time to wait, there are some advises regarding buying models right now if you want to start the army earlier:
- warriors - those guys are pretty much safe bet. There is a suggestion flying around they might (or not) have a new box, but I still think those guys will be our default troops choice - 24 of them is a good start (2 boxes). Also you will get some scarabs inside (3 units per box), which are really good unit choice these days. If you have our army starter box, you probably don't need more at the moment.
- Barge - right now and for next 3 months this is our best weapon. I would just not glue the gun to the Barge itself, to make it easily convertable to CCB. I don't think CCB will be affected by next codex as most of it's rules are in rule book anyway. And you will get a free Overlord/lord. And don't buy more then 2 of those.
- Nightscythe - even if they got nerfed Tesla and/or raise in price, you will still need at least 1 or maybe even 2 for transportation service - this is best way we have to our units delivered precisely where we want them. Also don't glue the portal, make it easily convertable to Doom Scythe
- Immortals - again as warriors, I think they will be as useful as before - I would glue them with Gauss, as Tesla is already a questionable choice (depends on personal preferrence) and it might be nerfed
- Crypteks - you can either buy those guys alone or buy a package of Lychguards/Praetorians and convert them (no special skills required really, I've done it), especially if you have unused Deathmarks head from Immortals pack. And 5 of those guys is not too many. I don't think they will be affected by much in new codex. Beware: there is a rumor (salty) that instead of codex we will get a box by the end of October with plastic Overlord possibly mixed with Crypetek. Which would be great, although not really cheap

Still conversion from Lychguards/Praetorians will be cheaper.
Those guys above will form quite a good army for a beginning which should still have a value after new codex shows up.
Ghost Arks - they are pretty good, but as "meta" changes and more people bring weapon to handle Imperial Knights and Land Raiders, there are slightly loosing their value. I have no idea if they will be touched in new codex - should be ok if they don't raise the points cost.
Doomsday Arks - those are poor choice right now (unless you have very specific setup = landing pad, stalker etc.) and I really hope they will get better in new codex. Right now I would strongly advise against it
Doom Scythe - easily convertible to Nightscythe, hard to say what will happen to it. Right now you rarely jink with it anyway not to loose it's death ray, so might not be affected much by possible Tesla nerf. I don't think they will be even more expensive in points.
Annihilation Barge - they will be nerfed. 100% sure of it. Just don't know how much and by what means
Lychguards, Praetorians, Destroyers - at the moment those are not really a very good choice to buy. First two are poor units (too expensive and not effective enough) and Destroyers are so-so, but might be redone in next codex. I wouldn't really buy them at this time.
Deathmarks - hard to say. I don't think they will be affected by next codex but huge part why we like them is how they interact with Despair Crypteks - if this interaction will be nerfed/removed, they will loose their appeal by a lot.
Monoliths - they are fun to use, but not really effective. I bought one because I can't imagine Necrons without it - it's just sooo Necronish

We hope it will be buffed in some way, it might not. We'll see.
Tomb Stalkers - no idea here. They are really a nice model, need very specific army to use them to fullest and are expensive in points. So right now not a good buy for a starting army.
C'tan - those guys needs buff badly, so don't buy them right now unless you get a cheap one second-hand

You might not use them ever.
Spiders and Scarabs - you can't buy scarabs, they are inside warrior boxes. They are great units right now, especially in specialized build with spiders. I hope those will not be nerfed and we can still use them. They are not really that effective alone and of itself, but together they make such a wonderful combo.
Wraith - one of the best units we have right now in our codex - fast, deadly, pretty resilient. Especially if you add Destroyer Lord. But they will get nerfed - I think their price per model will raise and I wouldn't really be suprised if it were around 45 points per naked wraith. Which will still leave them very good CC unit, but not that point effective. As they are not cheap to buy, I would not invest in them right now.
Tomb Blades - on of our good units. Not a best one, has a specific use here and there, you also have to magnetize them (average in difficulty) in case you will want to change their weapons. I don't think they will change much but there is always a question where they will stand in comparison to other units after new codex comes. I might even buy a package just to paint them, I like the model.