I would like to share my experience as my mom accidentally sprained her ankle in Phnom Penh.
However, we still had a great time for Angkor Wat trip.
Though it's true that many sites are high with stairs.
Still there are many sites that are flat we could go.
Her condition was she couldn't really walk. She could walk with crutch but not for long.
It's easier for her to hold the wheelchair to walk.
Most of time she sat in the wheelchair and i pushed her wheelchair , but some parts she need to walk a little bit
Sometimes she would push wheelchair to assist her walk.
We would skip the sites with too many stairs that some you can tell easily from the entrance.
Or some sites maybe we just visit the flat parts and skip the parts that require to walk up.
I did find in web that there is a travel agency name ABOUT Asia could do disabled travel in Angkor Wat.
They even have people to carry you to the some high parts.
This is the link
https://goo.gl/RjWEjT
For us, we just rent a tuktuk and put the wheelchair and crutch with us.
It worked okay for us.
I couldn't recall the sites that we have visited, but i remembered more than 5 for sure.
We did go to banteay srei; and i agreed with previous reply nathalie2; it was a lovely place.
There were some stairs to climb, but not many, in general was flat.
I recommend that that you should leave early maybe 4 to 5am everyday since it is really hot.
Then call it a day early.
My friend told me that you can rent a wheelchair in Siem Reap for maybe USD 5-7 per day.
maybe you can check with your hotel or hostel.
Hope you enjoy the trip:)
Angkor Wat was gorgeous!