@Hein Rook > the 400 MT is much newer than the CDJ100 - for true comparison purposes I would encourage you to test out ANY new gear you may choose to buy. The successor to the 400 is the 350 which does incorporate a little more technology than the 400.
The CDJ-400 is compatible with most software suites such as Serato / Traktor etc. You can also load a bunch of tracks on a USB stick and play from that media.
This comes down to the laptop you are using as well as the software. Generally, you should be able to setup everything in such a way that there is incredible low latency (i.e. no lag / delay).
Again, this would depend on the software of choice and what you mean by "all functions" but the general answer would be yes for the most part.