If you're looking to tune down long-term, and want to set your bass up accordingly, I can't recommend Newtone Strings enough.
When my band FSOF was playing in Drop C exclusively, I had two basses set up in Drop C standard (C, F, Bb, Eb) with their High Tension strings. The extra tension meant that I could have my strings in normal guages 45-105, and a low action too, nothing flapping or buzzy!
Just to re-iterate, I used 45-105 guages in Drop C Standard with pretty normal action.
The only negative with high tension strings is that Newtone taylor them to the tuning you want, so although you'd probably be able to take them up to Drop D standard, getting them to E standard would probably make them too tight to play comfortablely. This obvisuly isn't an issue if you're in Drop C all the time.
Si