I'm assuming you want to know if A5 can be used to make a game like DOD, and not specifically a Half-Life MOD? Yeah, it can. You can make any type of game your mind can conceive and talent can handle. If the talent is there, it will look as good and, hopefully, play as well as those other more expensive engines.

You can certainly exceed Half-Life quality, though I wouldn't expect you to come near Doom III quality, at least, not yet, anyway. When all of the experimental features and DirectX 8.1 conversion come around, then we'll see. Of course, it will take quite a bit of talent to get to that Doom III peak.