Start with one real customer journey
Map how a lead becomes a customer, how ownership changes and where information is copied manually. Include the exceptions: a customer with multiple branches, an unusual approval or a reopened deal. Test this journey in a SaaS trial before writing a custom specification. A platform that handles your normal work with modest configuration may be the better starting point.
Compare the whole operating cost
SaaS costs can include seats, storage, API limits and paid extensions. Custom software adds development, hosting, maintenance, security review and support ownership. Build a comparison over a planning period that fits your business. Include migration and exit costs on both sides. Do not compare one month of subscription against only the first development invoice.
Treat integration and exit as features
Ask whether records, attachments and relationship history can be exported in a usable form. Verify documented APIs and how rate limits affect synchronization. For a custom system, require a data model, backups and a handover plan. Owning source code is useful, but it does not automatically give your team the knowledge to operate the application.
Choose a reversible first step
A hybrid approach may fit: use SaaS for standard sales activity and build a small integration for the workflow that differentiates your business. Run a pilot with a representative team and define what would justify further investment. The decision should be based on observed friction and operating needs, not on an assumption that custom is always more flexible.