It's a waste of time and money don't bother
you cannot unlock them you have to buy them on PSN or LIVE Marketplace
Yes, you can.
You can buy each 5 of them for 14.99$ on steam
you can't. you have to buy them for 1200 Microsoft points or $15
As your spec ops level increases you will unlock the different map tier's, to do this play the easier maps first.
It's all up to your personal preferance but I prefer the modern warfare 2 maps
No the maps are different
The Modern Warfare 2 stimulus package is a set of 5 new multiplayer maps downloadable for PC, PS3 and XBOX360. Three of the maps are brand new, the other two are remade maps originally found in Call of Duty 4: Modern Warfare.
The maps are sold at the PlayStation Store
buy stimulus pack
You buy them
What faction you play as is chosen randomly as one of two in a map. On Ranger maps, Its usually the Rangers vs. Spetsnaz.