Welcome to the Crons!
Thing about the crons that is very different to most
SM armies is that we have the ability to make competitive but very, very different style lists by selecting different units from the codex. There really aren't any "auto includes" which appear in every list, but there are "auto includes" that appear in each style of list.
For each style, there are many, many examples of lists available on the web so I won't provide exact lists. What style do you want play?
1)
CC?, then wraithwing is the go. Max wraiths, take an
HQ, min warrior squads in Night scythes. From a $ perspective, not cheap as wraiths only come in boxes of 3.
2)Mech? AV13 spam for you. Ghost arks, Annihiliation barges (even a stalker if you want).
3)Foot? Blobs of warriors it is. Support with a ghost ark or two for regeneration, lord with res orb and away you go.
So, you need to pick a style first. Then, there are some units which are generally better for the points (these don't fit into every style list, but mostly)
-Anni Barges : your heavy support unit of choice.
-D&D squad: Deathmarks, with a DespairTek (Harbringer of Despair - for the AP1 flamer). You need a veil of darkness or a Night scythe for delivery.
-Wraiths : Your
cc alternative for a lot of lists. Take in groups of 5, 2-3 whip coils.
-Crypteks : Favourite flavours are
1)Harbringer of Despair (DespairTek) for the AP1 flamer, include veil of darkness (VeilTek) for added transportation fun.
2) Destructek (HoDestruction) for the lance, with pulse sometimes (PulseTek)
3) StormTek (HoStorm) for the haywire, plus lightning for
CC protection (LightningTek)
-Zahndrek : Great force multiplier because you can take some special rules away from an opponent, but also give some to one of your units. Scarabs with furious charge? sure. Wraiths with stealth? Done.
There are others, but this is getting long.
Final point on $$. I'm assuming that you don't have any models, so your first place to go is the battleforce. However, from the battleforce you can make a lot of things (eg crypteks, lords etc) by adding in spare bits from a lychguard box. There were a couple of great Dakka threads on what to build from the battleforce last year. If you can't find them, give me a shout.
If it sounds complicated, it really isn't as long as you pick your style first.
Then, Dakka will provide

!
edit for formatting