My guess would be: basic rules first up with the basic LoS and UNE figures. Knowing all the weapons rules is a big deal with this game so I'd limit it to stuff that sticks with the common rules. My starting-point is the Black Box as I never saw the Blue Box, but for a download you don't want to be that sparse. Hint at the more expansive UNE and LOS lists, add some stuff that spices the game up without adding too many extra rules. Yes, all subjective points I know. (Ideally you wouldn't overwhelm anyone, wouldn't disappoint anyone, and would leave everyone looking for more. Never going to happen. Just the ideal.)
Then I would imagine, Advanced. Advanced and optional rules. This should basically be a bit of a rules-junkies' paradise with lots of options and tweaks, and guidelines for introducing rules (including "by mutual consent only" for the far-out or very detailed stuff).
Lots of possibilities here, one of which is to bring in all or some UNE and LOS (depending on the next step, see below) and IMO, the basic squad for each of the Alien empires, with a very short blurb on who they are and what they do. This would be good if the aliens weren't back in production yet but/and you could get their basic troopers out soon. Possibly keep the Angel, T85, ASP and maybe Mk3 for the full army lists. The Angel was part of the backbone BE boxed set, but I think she might make the BE "book" a let-down?
Then, IF you go with downloads, you do pretty and complete army lists one at a time for the "Aliens" AND for each of the UNE and LoS. The trick is to take full advantage of the format and make something very nice, and very cheap. Maybe the LOS and UNE should be half price, since your customer has already paid for them in part. The UNE and LoS would of course be complete, including all earlier stuff for those armies.
I think people will feel they're getting value as long as each chunk is cheap enough and gives them stunning new art to ogle, and of course new tactics to cause their opponents some grief.
In summary: rules, then optional rules, then army "books".