Signing another center and letting Sullivan walk would be a HUGE mistake for the Rams. Rams had one of the best OLines last year and were the only OL to start every game (week 17 doesn't count, they sat some of them on purpose).
Sullivan called all the line play, is a savy vet who knows all the D tricks and stuffs the middle with the best of them. To bring someone else in would disrupt the intire OL continuity. You don't phuck with what works.
Rams will find a way to re-sign Sullivan if they're smart. He's worth more than what they paid him last year.
I'm guessing they get their DT in the draft. Starting a rookie in between two Po Bowlers makes sense and saves cap space.
Don't know whats left at LB or DE, but using whats left of cap space there makes sense too.