You should do both. I have found lots of gently used Halloween costumes or dance/recital costumes at thrift stores. There are also the cheap worthless ones too.
As for accessories i would say if it goes on the hair to buy as new as possible.
Think head lice...I know, you just cringed. It happens. Sometimes when a person wears a hat or crown and shares that the next person may or may not pass on an uninvited guest.
In my opinion, after 13 years + in child care, that these areas are the best and most important areas of children's playtime. They use total imagination, they can find things to do for hours with a well fitted out pretend play area.
So, I would buy a few items that are ladies size small or things that would be totally fun. Like huge hats, high heels, beads, clip on earrings, brooches, bangles, necklaces, glasses without lenses (you can pop them out), etc...then I would buy some quality dress up clothes in the thrift store that would fit. Long dresses, crowns, wands, tea set, dishes, fake food, shopping cart, stove and kitchen area stuff, so many areas.
I think you sound like a fun friend for this little on to have. You are thinking of something that will be the best thing ever. She will still be playing with these items the day of her next birthday too.
Here are some links to help with ideas:
This is a link that talks about what pretend play does for a child.
http://www.scholastic.com/resources/article/the-importanc...
This link is a mecca of ideas, it may help broaden your scope or even fine tune your idea. The links on the left give different areas of pretend play and each one will have dozens of ideas in it.
http://www.toysrus.com/category/index.jsp?categoryId=2256395
This one is Little Tykes. I added it because you can narrow down the ideas by age. Their categories are more fine tuned and can give you even a better idea of things specifically for her age group and how it will lead to the next stage of development.
http://www.littletikes.com/pretend-play