First of all, it's all about what you like best yourself. Go to a store and try out those players and mixers. Its really personal what is best. The advantages of a cdj 400 to a cdj 800 are: -the (scratch) effects, -the USB port for music, -it can be used as a midi controller, - it's small (the same size as the cdj 200) so you can easily take it on the road. The only thing that you can make doubts of is the sie of the jog wheel, the cdj 800's jog wheel is just biggger and some people thik it's easier to work with a bigger jog. * I will have some cdj-400's soon, that's my choise*
The mixer is personal once again but there is a major difference between the two mixers; 4 channels or 2. If you're planning to just play with two cdj's buy the djm 400; it has most of the features of the 600 but it has an inloop sampler too. It also costs around €400 less. the djm 400 cost around €500 and the 600 costs around €900.
Hope it's abit of a help!