What to look for in an isolation transformer?
'Isolation' means different things to different people, there are different grades of it.

The differences come in two areas. Safety, and performance.

Safety. You do not want the isolation between primary and secondary to break down normally, but how much abnormal are you willing to tolerate? 1500V is a typical level for industrial and mains, though you might want to specify 2500V, or 5000V, or some other higher figure, if you felt the cost of a breakdown, perhaps the patient's life in the medical case, was very high.

Performance. Ideally, an isolation transformer will no connection between primary and secondary. Imagine a motor driving a generator on the end of a 3m long plastic shaft, that would be very isolated. In practice, there is some capacitance between all of the primary and its wires, and the secondary and its wires. You can model the effect of that as an AC current source at mains and its harmonics between primary and secondary. For an industrial application, that might not even be specified. For a measurement or medical application, it might be specified to be below a maximum measured in the uA or even nA.