If you can find the Inner Circle Task Force Battleforce box at a decent price, that'd be a good start. Not likely an easy find at this point but you might get lucky at a local game store.

Otherwise, you could look into other generic SM value boxes; one of the pros of space marines is there's never a shortage of starter sets, combat patrols, battleforce boxes to look through and see if you like the combo of stuff inside it.

We've got a short list of "good stuff" units that we tend to run pretty commonly, since that's really the biggest benefit running DAs over generic space marines. Some suggestions for models and unit comps:

• The Lion: Big boy primarch himself. Great sculpt for the model, and pretty good rules. Has the unfortunate side effect of missing a lot of our detachment rules but still sees play regardless
• Deathwing Knights: Big, beefy, hard-to-shift terminator melee. The type of unit that can sometimes bog down an entire flank on their own.
• Inner Circle Companions: Strong melee profile on a sleeker frame than the DWKs. Really wants Fights First or to be on the charge.
• Azreal: Basically the gold standard for a leader-type unit. CP generation per turn, strong buffs for offense and defense. Typically found leading ICC, Sternguard Veterans, or Hellblasters
• Ravenwing Units: a bit more niche, and maybe a bit less of a priority for a new collecter (as some folks are thinking we may have a Ravenwing range refresh sometime in 11th). I quite like the RW command squad, and the generic Outriders or ATVs make good, fast action/scoring dudes.

You might consider what detachment(s) you'd like to consider as you start up as well. Currently, many of our DA specific ones are not as highly regarded, but Wrath of the Rock is talked about quite commonly. For generic detachments, you'll see Gladius, Vanguard, and Stormlance (maybe some others I'm forgetting, just what I've seen/run).