There's a thread that suggests a range of basic E:A lists for 3k point tournament play: IIRC half of the points for the the IG list were 2 companies infantry, 1 command company, 1 Leman Russ company for 1600 points, then a range of other units.
Nealhunt (or dave) mentioned that victory in E:A generally is decided by control of territory -- objectives, etc.  This is a good omen for the Queen of Battle.  Indeed, there has been some discussion of Ork tactics that suggest the value of using an infantry cordon to minimize the enemy's mobility... in which case, each infantry stand's ZOC may be its most valuable weapon.
And, finally, I might suggest that you consider using Dark Realm Miniatures or Exodus Wars miniatures for infantry:
www.darkrealmminiatures.comwww.exoduswars.comDRM's cost is 4 pounds for 40 white metal Ar-*adian infantry, who have shoulder pads and helmets that are a passable likeness to some WH4k infantry I've seen. My impression is that this is a pretty good price point... Of course GW would likely enforce a 'no non-GW proxies' rule in the E:A tournaments... if it ran any of them.