Most of what I would answer is similar like
tanoshi's answer

1. Yup, very convenient, because it is in central Tokyo itself.
2. If you compare using the Subway vs the JR Lines (which is the normal trains), then the answer is Yes, subway is expensive.
As suggested by tanoshi, try planning your destination using
http://www.hyperdia.com/cgi-english/hyperWeb.cgi and
http://www.jorudan.co.jp/english/norikae/ .
3. It depends on what you want to eat.
Fast-food restaurants are everywhere.
Convenient stores are even more (like every 500m, you can see another one).
Japanese food are aplenty.
I suggest you plan it all out first by doing more searches on the Net, including what you prefer to eat.
Firedauz, is it possible to live on onigiri (breakfast), bento set (lunch) and fast-food ramen (dinner) for 5 days ?