Hi Thomas
September is a good month to go as the temperature is likely to be pleasantly warm - in the low 20s. I visited in early October once and it was still nice then, with the odd shower!
The currency is Croatian Kuna and the exchange rate is around £1 to 8 Kuna.
I think you will feel very safe there, as the people are friendly and there is a very laid-back atmosphere. If you take the same sensible precautions you would take anywhere else, I'm sure you will be fine.
The towns you will be visiting are very pretty and a joy to walk around - Rovinj was my favourite.
I hope this helps.
Kind regards
Olly, your editor