my recommendation is to use the Unreal engine. I've seen some very realistic mods done on it (check out Infiltration for Unreal Tourney http://infiltration.sentrystudios.net/). Pick up a copy of Unreal 2004 DVD Special Edition. It comes with the Unreal Editor as well as a second DVD dedicated to teaching you the editor. You should be able to find it new at your local electronics Giant (like Best Buy, Circuit City, EB, Fry's Etc...) for under $20.
Games may be useful for some training (they are great for developing Hand-Eye coordination), but a Sim is a Sim is a Sim, you know? I know I have a blast playing them; from the fantastical (Doom3) to the ultra real (like Infiltration).