Each of the two major expansions fixed many of the problems present in the previous version. So, if you buy anything, definitely get both expansions as well.
Personally, I think the biggest fault of the game was the 1-unit per tile, and that hasn't changed, but there are many people that like it, so YMMV.
As it is now, with Brave New World, it is playable and enjoyable. There are balance issues, but you can play on high difficulties with several different strategies and all civs, so it isn't nearly as broken as it once was.
$12.50 is not a bad price for the game, if you like Civilization games at all.