Should I be shooting this @ ISO100 rather than ISO160? Image a bit dark??

I would have just placed an incident light meter in front of the model, pointed toward the camera, using a reflector to bounce sun light on her. The incident metering will place the skin tone just right and make the backlit hair "shine" which I suppose is the intended effect (not entirely reached, which makes me think the exposure is actually a bit too scarce for the shot). The dynamic range of the negative colour film would hopefully retain the detail on the vegetation behind, which should be around 2.5 EV above the shade and well inside the negative film comfort zone.

Personally I would place the model hands differently so as to avoid the risk of blowing the highlights on the hands. If the hair have to "shine" in contre-jour, they should be the only part directly hit by sunlight.

Is it just me, or is there also a slight green cast to the overall photo? The tonality seems to be lacking, how do you process these? I could be totally mistaken but just an observation that as well as other parameters the processing should be looked at too

There could be some greenish cast visible on the model complexion. I think this is due to the reflection from the grass. The contribution of the fill light should be increased in order to address the problem IMHO.

The green cast can obviously be also due to some scanning or post-processing problem. With slide film one might observe the slide and see if the cast is there. With negative film the answer is not obvious.

Another source of green can also be a gold reflector.

I have no idea why.