Stick with LB's if you can. If your league requires defensive backs and linemen then fill your linebacker slots first, then the DB slots, usually with safeties if you can (they get more tackles typically than corners) and last of all d-linemen. The best linemen will only get 40+ tackles while the best LB's will be at 100+ tackles. There are some safeties out there that break the 90 tackle mark as well.
Fill your starting offense first, but wait on the kicker until the final round of the draft, then make sure you have backups at key positions (RB, QB, WR) before you fill out your defense. The nice thing about IDP's is that there are usually plenty to go around. I was in a 20 team IDP league last year that started 2LB, 2DB, 2DL and 2Dflex. There were still decent players to be had even after all of the teams had their rosters full.
There's a pretty good IDP ranking here: http://www.fantasyfootballcafe.com/2011/player_rankings_idp_2.php