Regardless of the size always go with 300 dpi. If you are working on digital content then you could go with 72. I personally prefer 300 no matter what.
This is stander amount. If you want to print in high quality you can use more. But it increase your image size,if you measure it in inch or cm.
Yep. 300dpi is minimum for printing. And remember for CMYK palete. If you convert rgb to cmyk later, thean you will have some errors or difference in colors.
72 dpi would look very grainy and low detail. The quality, in this day and age, would seem very bad. I suppose it depends on your needs.