Well, theres a great deal of freedom with the IA (just as there is with the IG in M40), so you can pretty much use the codex as is (check the PH Tanks thread
here in PH Q&A to see where we are with detremining which tanks were available).
To me, the main point of a PH IA army is trying to get the look right - with my Surrians I'm going for greatcoats, shiny helmets and breastplates to present the kind of "good first impression" look that I imagine the expeditionary armies having - to me its less about the practicalities of camouflage, more about looking striking and important to wow those recently discovered cultures.
Commissars were known as "Discipline Masters".
Preists might be less present (depending on how ingrained in the Imperial Cult you see your force as being).
Hellhounds, if I remember correctly, weren't available, but I think the rest of the tanks and vehicles are ok (as I said before check the Preheresy Q & A thread for more info).
Land raiders were used by the IA, and we think rhinos were too (but we dont have a source yet). Not sure about landspeeders and jetbikes at the moment - although they were there in Roguetrader, I dont think we have any preheresy sources for those yet.
Mutants and abhumans were present, as were "grav vehicles" (ref: IG Codex) though other than the classic deoderant stick grav tank depicted in the CCG, we dont know what these might have looked like.
I believe Legion (the HH novel) is apparently a good source for IA info, though I havent read that far yet, so cant confirm whats in there. I believe that whatevers in there is representative of a particular regiment, so while you can take it as indictaive of what was available to IA forces, it doesnt mean all regiments were built that way.
As for a lascannon IA army, the way I see it is that different regiments have different specialities/themes. Each expeditionary force would have multiple regiments on board, as well as various individual units seconded from other fleets/picked up along the way, so when the expeditionary force goes to war it can pick and choose which elements it will field from those available to it.
Therefore, you could have an all-lascannon heavy platoon in a mixed regiment army if you wished, and say that for fluff purposes it comes from aa regiment that specialises in lascannons.
This is what I intend to do with my IA - the VIIthSurrian Janissaries are an all cavalry regiment, the XIVth Dreahn Abhuman Assualt Auxiliaries are 90% beastmen (led by human command squads and discipline masters) plus I'm brainstorming ideas for a regiment based around heavy weapons. All three regiments might be represented by units if I took the army to the tabletop, providing a balanced force, or I could go for a specific regiment if I wanted to challenge myself with a very restricted formation (although thats when doctrines come into play). If you havent looked already, check out the
Members Blogsthread where you will find a couple of expeditionary force threads (mines the
119th) which might give you some inspiration.
Hope that helps.
-Anodyr