Bariatric is just a small part of ESS. ESS involves high acuity transfers and now includes response to category C "emergency" work that doesn't need paramedic and lights and sirens , they also are doing more and more lights and siren first responder duties (respond and provide care until paras get there) they also do mental health transports.
So to do ESS normally the AOs spend some time in PTS if they are good they get selected to do ESS, Those who want to apply to go from ESS to bariatric or mental health teams,
You just need a standard (car) licence + SAAS driving course to do general PTS/ESS. If you go to bariatric SAAS pay for you to get LR licence.
Regardless which area the qualification you have to do is Certificate IV in Health Care (Ambulance), the same qualification that SAAS country volunteers do.