Some of the different clans offer a completely different experience providing some serious replayability. Playing as a Malkavian or a Nosferatu is a completely different experience than playing as a Ventrue or a Toreador. I'd actually recommend that anyone playing the game play it at least twice, once as one of the more "normal" clans, and once as a Malkavian.
*snip*
A quick google search turned up a fairly number of texture replacers and the like for Deus Ex, but I've never used any of them myself so I can't comment on their quality. With regards to mods there's a rather large community patch for Bloodlines that fixes up many bugs that were unfortunately left in the game upon release. There also seem to be a fair number of additional mods for both Bloodlines and Deus Ex, although again I don't have any first-hand experience using any of them and your Google-fu is probably just as good as mine so I'll leave further investigations to you on that matter.
I'd recommend that as well, though I'd take one of the normal clans for the first playthrough. If you take a Malkavian for your first character, there's a good chance that you won't have the slightest clue about what's going on

. And though the difference isn't nearly as great as with a Malk or Nossie, I found my Tremere playthrough to be quite different from the others as well.
As for mods and stuff, you should at least install the official patch, or you may run into a literally gamebreaking bug at about 75% of your game. I really like http://www.patches-scrolls.de/vampire_bloodlines.php as well (in which the official one is included) though it changes some dialogue, quests and other minor gameplay things as well besides fixing bugs. For the better in my opinion, but you may want to wait till your second playthrough with that, so you have had the 'official' experience at least once (yes there will be a second playthrough, I know you'll love this game

).
For larger mods, the only one I've tried is the Camarilla edition. It changes feeding and discipline mechanics. Was quite fun, but I would recommend against it for your first character.