A Custom Plugin Is a Permanent Commitment
This is the part that is usually left out of the sales conversation, so it goes near the top here.
A public plugin with thousands of users gets patched when WordPress changes, when PHP moves version, and when somebody finds a security hole. Yours does not. Whatever we build for you is code only you run, which means only you are responsible for keeping it working — and the day it breaks is usually the day your hosting upgrades PHP without asking.
That is not an argument against custom plugins. It is an argument for keeping them small, for insisting on documentation, and for budgeting an occasional review rather than assuming it is finished forever. We write to WordPress coding standards for exactly this reason: the less clever the code, the longer it survives.
The Test Worth Running First
Before commissioning anything, look hard for an existing plugin that does roughly 80% of what you want and is actively maintained — updated in the last few months, a decent install count, and support questions being answered.
If you find one, the cheaper path is almost always to use it and have us extend it, rather than rebuild the whole thing. If the only candidates are abandoned, enormous for the one feature you need, or priced per site in a way that scales badly for you, that is when custom genuinely wins.
The other strong case is integration: connecting WordPress to a system nobody has written a plugin for. That work only ever exists in your business, so nobody else was going to build it.
When It Is Bigger Than a Plugin
If what you are describing has its own users, its own data model and its own reports, it may not belong inside WordPress at all — that is custom web application development, and we will say so rather than squeeze an application into a plugin.
For plugins we build or inherit, keeping them alive sits naturally inside WordPress support or a monthly maintenance plan, which is the honest answer to the commitment described above.