The HK VP9 may be something to look at as well. It's got a pretty good trigger to begin with, and the grip is modular from the factory. There are quite a few options for triggers, sights, and other aftermarket parts available. HK is even getting in on the action with barrels and slides. Plus you now have a choice between a paddle or button magazine release.

All that said, my VP9 is bone stock except for the sights. One of my USP9s, however, has had some work done and is one of my favorite pistols to shoot.