do the research..
if you have a good computer performance i3/i5/i7 do CPU mining; if you have a good GPU do the GPU mining.If you have good CPU with GPU mine with GPU&CPU
Yes, but check your electricity prices first. It might be the case you are mining at a loss, so buying might be cheaper
People stopped mining because they where paying more in electric than they got in Bitcoin. If they had the sense to realize Bitcoin would go up in value, they would of made a profit even they they where loosing at the time.