I'd rank ACE and AFAA certification on the same level as the weekend courses, unfortunately. Now, there are trainers with those certifications who are highly qualified, but make sure you check them out thoroughly if that's all they have. With your history of injury what I might do is ask at a couple of your local physical therapy clinics. A lot of PTs do personal training for not much more than you'd pay in a gym or studio setting, and one would hope it would be safer. If not a physical therapist or PTA, look for someone with ACSM certification.