Do you have the option to take her to a store and try some out? Somewhere like John Lewis or even better an independent. It's worth seeing how the seat fits her and which ones are comfy for her, since she'll be using it for a long time hopefully. You might also want to try them out in your car since it's a pain if you find that the seat base covers up the seatbelt buckle and you are always digging around for it or something. She can try out fastening it herself if you want as they tend to like doing this. Also, some of them are wider in the head area than others and can cause an issue if the sides of the roof of your car are curved as you might not be able to extend it to the full height. Only an issue if you think you're likely to have the same car in 5 years' time.
The Cybex, Nuna, Britax and Recaro ones are the current best rated ones for safety, comfort and ease of use and the Joie i-Traver is worth a look too (the cheaper Joie ones are a bit less comfortable). Isofix does make them a bit more stable which is handy, but doesn't make as big of a difference here as it does at earlier stages.
When you do try it or fit it in your car, it's best if you can take the headrest off the vehicle seat and insert it again backwards so it's flush. If your car headrest doesn't do this, the next best thing is to look for a HBB where the headrest of the HBB itself sits a tiny bit further forward so you can retain the support of the car headrest behind it, if this isn't possible, you can take the headrest out completely to avoid having a gap behind the child seat.
You can also extend it all the way up to make space but don't wedge the child seat in with the headrest as this can adversely affect safety 