"Isolate" usually implies that the two systems may have different common or ground connections. If your step down is done by a tranformer with no connection between the primary (230 V) and the secondary (6 V), then you may be able to use it without further protection. One key point to remember is that standard power transfomers may have significant capacitive coupling between primary and secondary windings and may have a relatively low insulation voltage rating. Thus coupling of fast spikes or high voltage transients (such as from lightning) may still pass enough voltage and energy to damage the DAQ device or transformer or even injure personnel.
Resistive or capacitive voltage dividers are probably not suitable because of grounding issues.
Voltage transducers usually have an isolation rating. If it is higher than the likely transients in your system, then such a transducer would be suitable.
Since you said you are a "starter" I highly recommend that you find someone with the necessary experience and knowledge to determine if your configuration is safe and appropriate.
Lynn
