assuming mm is memory

"i want to read data from a bar code reader and in the end of the day i'd like to remove it and connect to a computer, to transfer the read data."

2 ways:
1) develop a PCMCIA interface and use flashdisk cards (you do not want to do that)
2) make the it you remove the whole thing and transfer to the PC by serial(you will want to do that)

"First i thought in using a flash mm, because i've think it met my needs."

It will:

Wich mm should i use?
Rather than 8031 use a uC such as P89C51Rx2, this will relieve you from the bitchy hardware you need in order to share a flash for code and data. If you can live with 48-64k of flash you are then home free, just connect a flash as regular data memory, create a "write flash" routine that does a MOVX, this will load the ALE latch, then MSB of address on P2, data on P0 and drop !WR for the time the flash requires. If this is too much, use battery backed RAM (Dallas is a good supplier) this you work just like memory (just MOVX). Many flash memories will erase if you send a very unique sequence of writes, possibly with one bit set.
If you need more than 48-64k of data store you will need to do banking (use search), which can be done with either of the above.

Next time, stay away from "in house" abbreviations, I am posting ASSUMING you mean memory.

Erik