WordPress Developer Ongoing Retainer Postion

Looking for UI and UX developer to maintain, and do periodic development tasks on our established WordPress site. Payment will be on a retainer basis, irrespective of the amount of work that will be disseminated each month. Some months there may be nothing, other months there may be a bit more. All tasks that go out will have deadlines, but if the developer is having any problems in meeting those deadlines that communication is essential. Some development tasks will be disseminated in the interview process to determine the turn around for each type of task, as well as some spots whereby we want to see the developer have a good sense of UI ad design.