Catpiano

1)  

Take the advantage for digital, try more and more!!  unlike the old days, you didn't want to try 100 pic on film ..  but digital, can try more! (one reason why people photography skill now, is much better overall)
Buy some photography books, or go so some website, such as FM etc .. or DChome.net ... if you perfer to read chinese. (some TW sites are good)

2)

Don't even ask this question, you shouldn't even ask if u need External flash, you should as WHICH one should you get ..

It is all about PHOTOgraphy ..  Lighting is everything!  Most important part for the camera ..  FLASH. (unless you only do studio pic, and using studio light! )

3)

Hard to say what is a good lens for you ..  or a bad lens for other ..   400mm F2.8 is a good lens for sport photographer, but may not a lens for you to use ..  
Try to take more pic, and see if you like to do more wide angle, tele, sports, wedding ..  whatever, Good lens for me, may not be a good lens for you.