Sounds like any Scientific civ could be up your alley. I'd say play around with all of them in turn until you find what you like.
The Byzantines can make the early contact (especially on archipelago) and trade their way to a science edge that the cheap libraries and universities will really drive home. If you're looking for diplomacy in particular, they might be the way to go.
The Russians can pop those huts and make early contact on a pangaea. Although you say you're a peaceful player, so the Russians may not be your bag. I always seem to start rather slowly with them, and catch up later via terrorizing my neighbors with Cossacks. Definitely give them a try, but you may not have as much fun with them as the others.
The Sumerians can get big cities fast and/or spawn settlers quicker to grab that land. I don't really like their UU much and haven't played with them much, but they do seem like they should be able to beat other civs off the line and stay ahead.
Babylon seems ideal for the civ-wide culture win, with cheap libraries AND cheap temples. I don't like them very much, but many people swear by them.
Greece and Korea can make more money and have less of it lost to corruption, fueling much research which the cheap libraries will amplify (the greeks also have that uber-defender of the ancient age, the hoplite, which will help keep more aggressive neighbors off your back.
Persia and The Ottomans, well, industious means you'll get your tiles paved with roads and mines and irrigation way quicker and/or with fewer workers, which will help your cities get big.
The Germans are probably the scientific civ you're least likely to enjoy, since you say you're a peaceful builder and the Germans' other strength is Militaristic. Also, the German UU comes very late (only the Yanks get theirs later) which means you'd be waiting a long time to flex that muscle. That would be when you're grabbing the most territory -- and at your neighbors' direct expense.
You CAN play a peaceful game with the Germans though, and that Scientific trait will serve you well to that end.
If you're going for 20K in one city, Sumeria, Persia, or Ottomans should enable you to get one city big and productive quickly, and start snatching the wonders with it. I've never won by space in Civ 3, so I don't know who to recommend for that; at warlord/regent I think any scientific civ can get you there without too much trouble. Diplomacy is all in your hands; you could make Shaka Zulu the Ghandi of your civ game, it all comes down to dealing honorably with the AIs and building relationships/alliances.
But yeah, the best way (and the fun way) to learn who you like playing with is to play a bunch of games, with various civs on various maps.
