Triton XL work class ROV. Not only has it been the workclass workhorse for quite a few years now but a lot of the newer ROV’s are based on it with obviously some upgraded bits.
However for a worst case senario and to really have the trainees pulling hair out get an MRV.
Subserv used to have a little eyeball Apache vehicle run out of a van which for a training tool is pretty good. There are a lot of different systems out there and ROV introduction courses can only ever really be just that, an introduction keep it simple and straight forward. Get the basics right: fault finding techniques, spacial awarness, safety and attitude and the rest will fall into place.