First: Welcome to DJR!
Question 1: There absolutely isn't a best combination of gear, because everyone likes his own things, some people may like the functions on Denons more and some like Pioneer more. There is a standard club set: 2x CDJ-1000 combined with a DM-800, but this will probalby change this or next year to: 2x CDJ-2000 combined with a DJM-800 (maybe combined with a Denon DN-X1700)
Question 2: That depends on what you're going to use it for, at home you won't need an 800 watt amplifier

And if you're going to use Active speakers you won't need an amplifier at all.
Question 3: Actually it doesn't really matter, but I prefer Active speakers cause you have less stuff you have to take with you if you're going to a party, also it's much harder to blow Active speakers because they always have an almost perfect amplifier in relation to the speakers themself build in.
Sorry for my bad English (if it is, I don't know?

)
Je kan hier trouwens ook gewoon Nederlands praten.