1. Is it possible to upgrade the RAM and use 4GB DDR other than 1333 MHZ PC3-10600S ?
• No Macs are sensitive to the speed. Even if you did upgrade to faster RAM, it wouldn't run faster. You can upgrade to 4gb or 8gb chips.
2. Is the serial number important ? Do I have to get the exact same ?
Serial numbers do not need to match.
3. Do i have to use the SAMSUNG brand or any other brand like Crucial is compatible for my mac ?
Brand doesn't matter, but I've seen issues with some Corsair chips. Crucial is a good brand.
4. Can i Replace CORE i3 by CORE i5 ? Is it compatible with the initial motherboard ?
@danj should answer the question. Simple answer is yes, you should be able to replace the i3 with the same generation i5 or i7, given that the CPU is not soldered on to the motherboard.