Is she self funding? This will give you much more flexibility.
My experience is that the management team is extremely important. Where my father was initially, the senior management changed and the level of care deteriorated rapidly.
I was fortunate that a place was available elsewhere so I was able to move him.
The nursing element is downplayed. On site nursing is important in dementia care, especially when it becomes advanced and there are co-morbidities. I would suggest you start with CQC Oustanding nursing homes first and don't go past any that have any rating lower than good.
When you visit observe closely and don't ignore your gut. Try to visit at mealtimes, are they able to offer individual help with eating?
Mealtimes and early morning are stress times for staffing. The final home my father was in had an extra morning shift specifically so there were sufficient staff members to get everyone washed, dressed and ready for breakfast. Observe the day rooms, is a staff member with residents at all times?
How do they deal with fall prevention? My father fell at least three times and the fall alarms didn't work. I suspect more falls, but was only told when paramedics were involved. When he was in the Outstanding home, he didn't have any falls.
Him being in the wrong home was even more stressful than caring for him 24/7 at home.