5 minute = 300 secs
300 sec * 50 read/sec = 15000

Thus with 16k of ram will be enougth

16*1024 = 16384 bytes of ram

I don't have schematics of your project.
For conecting external memory and devices see the tutorial in this site. For using the A/D read its datasheet.

In this forum there are some post about the use of serial and interrupts.
Use the SEARCH buton and good look.