8 GB will be fine.
Apple's price is high.
Yes other brands can be used, google eshop.macsales.com or crucial.com.
You can upgrade Ram on your own.
It will not void the warranty, Apple has posted a HowTo for this. Check it out.
1. 16GB's Max.
2. Price varies, have no idea about prices in India. However, Mac's are very picky about RAM. Here's a good place to look for prices & brands:
3. You can put up to 16GB's in your computer, but 8GB's provides the best price/performance ratio.
4. You can upgrade on your own & it will not void your warranty. Very easy to do with the right tools. The link I provided you also has complete instructions with videos. They also tell you the proper tools you'll need.
I checked with the local dealers. Kingston is very cheep
Can you please check whether the RAM specified in the link below will suit my model (15 inch Late 2011 Macbook PRO) ?
I'm confused whether it is a PC or an Apple RAM.