Guide
Build and buy are two options among six.
Reuse, extend, consolidate and do nothing belong on the same table. Most requests fail the first test: the capability may already exist.
Translate the request into a capability. Then look for licensed products, internal systems and local tools that already claim that capability. Compare coverage, usage, ownership, known cost, maintenance, security and dependencies.
Only after that comparison should build or buy become interesting. A build estimate without a reuse check is not diligence. A vendor list without an ownership map is not diligence either.
Write down what you expect the chosen path to change. If you buy and still build, that is a later outcome — and it should embarrass the original assumptions, not hide them.