As this looks like a project that will continue for a while, id suggest an upfront payment for the build, then get them on a monthly retainer for the updates/service/training/etc.
Sure some months you will do more work than your getting paid, but there will be other months where its the opposite. That way its a guaranteed monthly income, no matter what.
Make sure tho you write up a contract stating what is covered in the retainer, anything out of that is charged at your hourly rate