Steam is awesomeness. You sign up for free and if you digital download your game it keeps track of all your games for you. Games take up a lot of memory so you only need to download the games your currently playing. Steam stores the games in the cloud on their servers and you will own the rights to all the games you purchase if your hard drive crashes computer gets stolen doesn't matter not all is lost just sign up new computer and re download games.It's awesome because a steam game has all the software patches and a whole global community that supports and even MODS the games. You can also trade games and gift games you purchase just like you could a hard copy of a game also steam games go on sale all the time your missing out on affordable new games You don't know PC gaming until you join Steam.
I have a gaming desktop and a laptop, both with Windows 8.1, and Civ 5 runs just fine although I definitely have to play the game at low settings across the board on my laptop. (Intel Core i5 Ivy Bridge, Intel HD Graphics 4400)
Looking at the hardware on the Surface Pro 2, it looks like you'll be able to run the game but expect low framerates if you're playing at 1920x1080. I've also noticed better performance running the game in DirectX 9 mode on my laptop as opposed to DirectX 11 so you may want to go that route as well.
It appears to install the expansion packs you don't have. The game I had in progress continued from where I had left off, although it was obvious from the opening screenshots that new content had been added overall. Hope this helps.