Contract Todd Hollenshead at id for Q3 engine licensing. However, the going rate for Q3 engine licenses was around $150K if I remember right, so unless you have a little cash to spend, you might want to look elsewhere. See http://www.idsoftware.com/business/technology/
There have been rumors that the Q3 engine will be GPLed fairly soon, so that might be a good alternative if you can accept the GPL terms.
I would guess you would need to contact activision for RTCW or ET engines.
Or consider if you can make your game as a total conversion mod, without the engine source. The gamecode is free for Q3, RTCW and ET. If you want to give away your mod/game, I would suggest using ET, since the game itself is also free to your end users. If you plan to sell your mod/game, you would still need to negotiate some kind of commercial exploitation license.
As far as engine capabilities, I’d suggest:
- look around at the q3 engine games/mods out there (being aware that many engine licensees have added features to the engine).
- look at the include files and *_syscall.c files to see what sort of features the engine offers. In general, the engine is quite low level and all the gameplay, physics and visual elements are defined in the game code and media.