I think that having MH issues, as long as you are stable and not likely to upest yr MH by the nature of the job will actually be seen as a benefit, as you will be coming with empathy and understanding. The recovery support centre I attend (for BPD) actually has a base of volunteers that are all previous service users, lots of drugs therpist have been peviously addicts but have to have been clean for a certain period (I think it is 2 years). I certainly want to investigate the volunteer route myself and have been told by the RSC that they actively encourage service users to go down this route, so having past issues really shouldn'y be a problem. Good luck