I don't know as DS stopped using the cot at around 8 months and he just slept on a mattress on the floor before going to a proper bed around the same age as your DD now. So it wasn't such a big change for him I guess.
I wouldn't bother with a bed rail. Yes, she may fall out, but she probably won't hurt herself and will quickly learn not to. And Trish's advice is good about the 'rules' for bed.

Anyway, good luck!