The Metrolink cars will be refurbished by a third-party contractor 'across the country.' I am pretty sure that Caltrain does not have the facilities or capacity to rebuild these cars itself.
Some cars may be available for use in a few months, but others will require up to a year for rehabilitation. They are Bombardier Bi-Level Generation II design, similar to Caltrain's existing Bombardier fleet.
In addition, Caltrain will also extend the passenger platforms at several stations to accommodate the longer six-car trainsets.
Caltrain news re: Metrolink car purchase