Hi,
"Trent reznor's sound" is made up of a lot of different synths and some pretty intense audio processing. At least that's how it was in the 90s. A mac, Pro Tools setup and this freaky bit of software called Turbosynth. As far as synths he had everything from an ARP 2600 to Waldorf Microwave. So it's hard to pin point exactly what he used on particular songs/albums.

Since I don't know what your budget is like I can't make any suggestions really. Do you want a hardware synthesizer that will do it all (or try to anyway). In that case check out Kurzweil 2600 (or earlier models like the K2000). V.A.S.T. is probably the most flexible synth engine. Doesn't quite take it far enough in any direction but tries to do it all and most of the times sounds better than the synths it is trying to emulate.

Software synthesizers is another great option. I actually use them more than my hardware synths. At least for production purposes. Still play hardware synths for fun though. Anyway, one particular soft instrument you may want to check out right away is NI Reaktor. I don't think there's anything it can't do. There are lots and lots of software synths nowadays. A good discussion for another time.

Hey, hope I didn't confuse you but in case I did simply Google every word that doesn't make sence.

I'm just scratching the surface here since I don't know much about your situation (aside from what kind of sound you're after that is).

-ED-