does she need a potty?
sorry to ask but personally I don't like them
SBJr never had one, just left him until he said he needed the toilet one day and he used the loo with no problems ever since. He had a seat on it to make it easier for him to sit on but other than that.
Personally I think its best to let them see you using the toilet and then leave them to make the connection when they are ready. My mum apparently did this with me and my sister as well. I have to say, it was very stress free!! He was just over two, had no accidents (except once or twice when we were out and didn't make it to a toilet
) and was quite straightforward. I think it was because he decided when he was ready and so recognised when he needed to go. My neighbour had a boy couple of months older than him and was training him for months and constantly having accidents and problems - all seemed too much hassle to me
They will do it when they are ready - you don't see many older kids wondering about in nappies 
I'd save your money and just let her see how the toilet is used - she'll get an understanding soon enough and saves you having to clean potties 