Ticket Plugin Required

I am looking for some advice on a plugin that will help me manage requests for work. The solution is for a small charity and needs to be free.

I started by looking at Support Ticket plugins but think these may not be a good fit for my needs. I think that some form of workflow engine would possibly be better and more configurable, can anyone recommend any good ones that would meet the following requirements for my system:-

1 Has 3 types of user, ‘Requestor – Person who requests work’, ‘Worker – Person who completes work’ and ‘Admin’ (All types of user will be registered in wordpress)
2 Requestor creates a new ‘request’
3 Admin approves a request
4 Worker selects a Request to work on