I've viewed a couple of videos on YouTube and noticed that you can build bases in DayZ. Anyone know if it's possible to do so in the original Arma 2 version or if that's perhaps some other mod I'm confusing it with?
It depends on whats a base for you, I'd say "Base building" in the "standard" ARMA2 DayZ Mod is not possible. You can place tents and Wire Fencing's and these kind of little things.
Building real "bases" is possible in the ARMA2 DayZ Epoch Mod (which is also available for ARMA3 btw), which you can find here: http://dayzepoch.com/
You can find information's about how to build a base on youtube and of course on their (new) wiki: http://epochmod.com/wiki/index.php?title=Main_Page
I'd suggest you to use the "DayZ Commander". It's a tool that let's you easily install several different DayZ mod's and has an integrated server list with many filter options. http://www.dayzcommander.com/
Base building is also planned for the DayZ Standalone (which can be found on steam), but the developer's are not sure when it get's added to the game.