How would you implement client, salesman, sales tracking system?

Will be helping a friend out by developing a business tracking system for his medical supplies company. The requirements seem very simple: the features that stand out are that salesmen shouldn't see others' clients and that the system would allow signing of pdfs by clients and storing those pdfs. We want to track clients, sales, salesmen, and when clients should be up for a resupply. Some notifications might be useful.

The obvious approach, for me, would be Postgres + some widely used web stack, everything custom. However, I feel like this use case is a very minor variation on a very general use case of tracking a sales-based business, and am wondering if there's any products, selfhostable or not, that I could customize instead. How would you approach this?