I agree with MMM and the others but I have always exchanged money in Japan at the airport or banks and generally get a good rate. The exchange rate is changing a lot lately so before you exchange anything get a good idea the rate to know if you are getting a good deal or not.
Learning some basic Japanese will also be very helpful. Do not worry if it is perfect.
To find out if the JR Rail Pass is worth it or not you need to calculate the costs of your transfortation. See
Japanese Train Route Finder By Jorudan Co.,Ltd. for an idea of costs and the like. It will also tell you if the train in JR or not. You can take JR and use your rail pass from Nara to Kyoto.
Kyoto is amazing and I highly recommend you spend a few days there. Nara and Osaka can be done as day trips from Kyoto. In Osaka I like Doguyasuji and Kuromon Ichiba. I would always take visitors to these areas and they always have a good time. These places are very different from what you will see in other destinations in Japan. American-mura and Shinsaibashi are also fun.
Spring in Japan is like everywhere else, some days are warm/hot and others are cool/cold.