What order do you suggest the expansions get added in? How is the quality of the cards?

There really isn't a recommended order in my opinion.  The mission and marine packs are small additions to the existing game.  The Tyranid expansion is more significant and adds some additional mechanics to the game. For that reason I enjoyed the added elements of the Tyranid expansion more than any of the other expansions.  The Death Wing pack was nice too, though.   Sadly, as with all POD expansions, the card quality is pretty poor, but that's how they are able to do these types of small add-ons and still make a profit.  If it's an issue you can always use card sleeves.

 I ordered the Marine pack already, waiting on it to arrive. I'll check out Tyranid next. Thanks

I ordered the Marine pack at the same time as the base game, which saved on popping out the team tokens (token sheet stored nicely back in the box, never throw em away!) and you get another 2 teams too!  Must admit that it wasnt long before I ended up purchasing all the PODs.

Quality of the cards is good for the price, just missing that textured feel to them (which you would when they are sleeved anyways!).

Best pack was Deathwing (not only for the DeathWing marines) but due to an advert card which read…

"You've protected the Imperium… Now protect your cards"

Best line ever!

Bottom line… you cannot go wrong with which POD expansion really!