Hi Rose,I did this trip with the add on in November 2018.
Walking shoes will be fine - most folks wore these - the ground was firm and dry on the walks we done. Boots would be fine too but I would not recommend sandals.
It was warm by day - I wore shorts and T's and 3/4 trousers when in city. At night it got cool but I would not describe it as cold. I wore fleeces and had a very light jacket for that extra layer. There are a few early starts so be prepared for fairly chilly mornings. Do not be put off by the early start for the monastery morning prayer - it's a highlight and a privilege. I wish I had gloves this morning as you are up early!
I would recommend poles - as Hils says, one is good but two is better. I was never a fan of poles as something else to carry but if I do another holiday like this, I would take them. A lot of people on our trip had them.
For the afternoon drive in Chitwan, take long trousers and long sleeved top and make sure they are dark colours - khaki, black or grey preferably. Take a warm fleece and or light jacket as it gets cool as the sunsets and the vehicle is moving about.
I know it's a small gesture but I took with me pencils and jotters. I took small supplies when on some of the walks as I knew we would encounter children in rural communities. They will be greatly appreciated. People often suggest you buy these when you are there to support local businesses (and save room in your case) but I'm not a great shopper so take these with me.
Just say if you want to know anything else.
Angie