To hire a right WordPress developer or company service, you need to keep a few things in mind:
Experience
Cost
Availability
Requirement Details about your project.
To know the developer is fit for your job you can have a skype call or chat with him, explain about your expectations and the end product which need to be delivered.
For development and designing you can either hire the WordPress developer or can get the fix cost estimation on the basis of requirement details.
Ensure the developer has good knowledge about the WordPress Website Development. If you hire WordPress developer on monthly basis you can use the developer skills for multiple WordPress Development tasks like:
-- Theme Development
-- Plugin Development
-- Website Development