You will enjoy this game. JO and JA are my favorite games still today. The SP campaign in JO is the best I've ever played. The SP campaign in JK DF2 is real close. The two best gaming experiences of my life were the first time I played JK DF2 and JO.
You should defenitaly get JA after you complete JO. The 6 level Dark Forces Mod for JA was just released. There are SP addon levels for both JO and JA to download.
My dream, to watch Imperial ships explode in my gunsights.