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Can't give in an in-depth reply, but quick overview of best to worst units in the codex.

Very good:
-Warriors (rapid fire buff combo'd with big Gauss buff)
-Wraiths (fast, large attacks/str, rending, fearless, great invl save, even better with a Destroyer Lord)
-Annihilation barges (very cheap for multi-purpose firepower including backup AA, quantum sheilding, and the model gives you an overlord model)
-Both Scyths (flyers are very good right now, both types work well)

Units to avoid (at least when starting):
-Flayed ones (more or less all around bad)
-Lychguard/Preatorians (too many points for a very specialized)
-Tomb blades (only really used for denying objectives, otherwise too many points for what you get)

I would also hold off on a monolith and ark (unless its in the battleforce) at first. They are rather expensive (both points and money) and I just don't use them very much unless I have a particular list in mind (and heavy support is filled up first).

TBH, the codex has pretty good internal balance, make sure you get it first before you start buying a bunch of stuff so you can see what stands out and what you like.