Buy where the workflow is well served
An established product may cover the required process with lower implementation and support overhead than a bespoke build. Check configuration, portability and the practical costs of adoption.
Connect where the gap is between tools
If the existing systems do their individual jobs well, the missing value may be a shared record, a status handoff or a reporting layer. Document access and ownership before assuming a connection can be built.
Build where the difference is valuable
Custom development is more defensible when a specific workflow creates commercial value and available products do not meet it adequately. Include testing, maintenance and eventual change in the cost of ownership.
Prototype before committing
Map the workflow, create a focused prototype and review it with the team. Use the prototype to test assumptions and define a first release rather than to imply that the production system already exists.