Client database and invoice system

We have a current database system in place which our training business uses to allocate tutors to students, keep track of students' lessons and payments and hold information on them. We need to urgently expand the system to introduce a 2-tier pricing system for students who pay in advance or arrears and add an invoicing system and add further functionality. We need to obtain a developer we can work with long terms as we may need support at short notice. The developer will need to speak English as we will need to speak directly about the project due to the complexities involved. I fully expect various problems to arise when we test the new system so please be prepared to spend some time after the initial work is done ironing these out.