Cobham started modifying their Dash 8s a couple of years ago so that they were capable of making air drops.
Wouldn't be hard to equip the Coastwatch aircraft with SAR gear just in case.
Reasonably simple matter to re-deploy a SAR equipped Coastwatch aircraft to a nearby SAR incident to begin a search and then organise replacements for both missions if necessary.
No need for a large fleet of specially modified SAR aircraft when there are already plenty of SAR equipped aircraft out and about around the coastline already. The new jets would be able to fill any gaps.
The existing Coastwatch Dash 8s have all the necessary search equipment fitted already. Add the SAR gear and air drop capability and there is a reasonably large fleet of SAR capable airframes and crews available.
Probably more than any other tenderer could have offered for the price?