Hello , I have a £2K bike or 2.....
Now until Christmas is actually an excellent time to buy bike, as new season bikes are released now (there's fashions in bikes!) and old season stock can have reductions of several £100.
The best bike is one that fits, and not all bikes fit the same, even if the frame sizes purport to be the same, geometry will be different and the bis will fit differently. Your DD needs to try a few out before settling on one.
Next most important is the weight of the bike. Steel frames and heaviest, then aluminium, then carbon, but if you are baulking at £600, then you are definitely in the leagues of a steel frame. These are not bad, but have been superseded by superior frame materials.
Lots of road bikes now have disc brakes. These add expense and weight, and you don't really need them.
Sub £1500 bike will probably have quite heavy wheels. Once I'd shortlisted bikes, I'd get the bike with the lightest wheels possible, as its the rolling weight of these that makes a big difference.
I would recommend looking second hand tbh. Once your dd joins a try club, I am sure she'll be swamped with offers of second hand bikes.
Or try Decathlon, they have the best bikes of the warehouse type shops and reasonably knowledgable staff.