The weather should be quite comfortable, evening may be a little cooler.
So just a thin jacket should be enough.
I usually recommend exchange some local currency (Czech koruna) before
the trip, enough for say 2 day's use. And then you can get additional cash
at local ATM machines.
Phone card I donno, I didn't carry a phone last time. I don't even know
what system they use, is it GSM?
7 days, you probably should go out of prague. I'd say prague can take
you between 3-4 days, and then you can go out to other citiies. If you don't
yet have in your mind which city/cities you want to go, try reference -
I don't have "things to buy" list for you, but I do highly recommend Prague,
I went 3 yrs ago, I love the city. Just soak in the culture and atmospher of
the Bohemian world!
Vitus Cathedral at Prague Castle is wonderful, especially if you can do some
study about the history of prague and prague castle before the trip, you see
much more by doing your homework first.
Have a great trip!
Myself, trip to Czech 3 years ago