A sort of cavalier or knightly type warrior seems good for Sturm. I say take a look at whatever PrCs that knights have, and have him follow the requisites as he wants to join, but he never gets the PrC itself. In the book he was only raised to the knighthood right before going to the Tower of the High Clerist, and that will probably only be what, the final module he's in?
Anyway, use that as a basic framework, and then fill in any skills and feats that are compatible or which compliment what he has.