It really depends how comfortable you are with cash games. If you've never grinded tens of thousands of hands showing profit, then it's probably a good idea to start at 10NL until you start showing consistent profit and are comfortable with your game.

I think $500 is a fine BR for 25NL. It was for me anyway. I rarely had more than a 2BI swing either way in a session. Hit a cooler, move down etc. Just don't play with scared monies as I'm sure you know...