Booking seats questions

Dear All,

we fly only very rarely and then usually only within Europe. We'll send out 16-year old daughter on an international volunteering programme to Cambodia. We plan to book an Etihad flight from London Heathrow to Phnom Penh with a brief layover in Abu Dhabi. We'll book the unaccompanied minor service.

Our daughter (flying on her own for the first time) would prefer a window seat.

a) Should we book the 'Value' option (+£15 per way) and pay extra for the seat? (Google AI said seats would be around $50)

b) Or should be book the 'Comfort' option (+£79 per way), but we don't need any of the other perks, it would be just for the included 'Free standard seats'

c) Or should we not reserve anything now, and then during check-in (24 hours before the flight or so) see what seat she gets assigned and, if it is not a good one, pay for a better one? But I guess that can backfire if the plane is full and there's for example only middle-seats left...

Thanks for any tips and recommendations!