We've often gone down the route of 'modify an open source project' for this reason. For the price of a developer, we get the product in perpetuity and don't hav

We've often gone down the route of 'modify an open source project' for this reason. For the price of a developer, we get the product in perpetuity and don't have these exact limitations you specified of headcount or worrying about usage. For almost everything out there, you can find a version of it that's been made available with source code. Granted, unless you are essentially a software company this is probably difficult to maintain, given the additional level of expertise required. Another option we've found works is buy, to just get started, and invest moving forward in our own project. Getting something off the ground quickly tends to help us understand better the requirements.