Linebacker is the way to start building a defense. Every great defense has at least one outstanding LB. That's why Texans got Sharper, he's one of the outstanding LBs. Just look at TB with D. Brooks, Baltimore with Ray Lewis, Chicago with Urlacher, etc.
If I was a GM or Owner of a expansion team, and had a pick of any defensive player in the NFL. I would go with Brian Urlacher, he's young and he's one helluva athlete.