Just purchased it and dont know whether i should get level 30 and then go do it, or if i can do it vice versa.

Depends what difficulty really

Broken Steel changes the end of the original game. You really can't play it until you finish the game. After that, do whatever you want.

so i can contiue playing the regular game even after broken steel?

Broken Steel works around the "you die" part of the ending, so yes. Afaik, you can do whatever you want after BS.

okay thanks, feel free to lock this now.