Hello Graham,
You've cancelled the other project but I wanted to let you know some information so I've decided to reach you this way. I'm not actually bidding on this project.
Anyway, building a custom PHP web application has its advantages and disadvantages you should know of.
Overall, for the project of such scope it's certainly a better and more suitable solution.
It's obvious that your project needs a very solid database, which means that this technology stack (PHP + MySQL) is perfect: it has very high levels of maintainability/ customization as well as scalability.
The downside is - building a web application from scratch is costly.
Knowing what you're looking for, I can give you a pretty accurate estimate:
normally, such work takes 3 months to build.
Of course, a good developer will offer you to split the project into iterations and probably prioritize - built the core features first - this way you'll be able to launch and start using the system earlier.
If you require more information or want to have some points clarified - ask me at any time.
Best,
Anna