There is about 96k available, take out say 6k for heap and stack (depends on your program) so maybe 90k of RAM for general storage.
Is that enough? If not you are sh*t out of luck.
1 Mio samples per second.
For how long? 90k will give you 90mS of storage.
Where does the free RAM begin?
The top of the heap. The actual address is of little use but can be found at run time if you really need to know.