Reinforcements would be a great buy if you can find an inexpensive copy at a local game store or on eBay. Until that happens, if you'd like a wider variety of units, you might consider the very inexpensive Ogre: Nightfall set from yonder website:
http://www.warehouse23.com/products/SJG7224
While the original "orange" map that comes with Ogre is great, I feel that the game really comes into its own when playing on more complex maps. So consider getting Battlefields sooner rather than later. The Battlefields rulebook is also the most up-to-date.