Is the DJM-450 good for a first club-style setup?

Yes. It gives you the familiar Pioneer DJ layout, Magvel crossfader and sound color FX in a compact two-channel body. It is a common step up from a controller for DJs mixing vinyl or CDJs.

Does the DJM-450 work with rekordbox?

Yes. Its built-in USB soundcard sends and receives audio to a laptop running rekordbox, so you can mix digital files alongside turntables or CDJs.

Can I connect a third or fourth source?

No. It has two channels, so you can run two turntables, two CDJs or a laptop plus one deck. Adding more sources means switching cables or moving to a four-channel mixer.

Does it have an effects send/return loop?

No. It has onboard sound color FX and beat FX, but no send/return for external pedals or rack effects. DJs who rely on outboard effects should look at a larger model.

How does it compare with the DJM-250MK2?

The DJM-450 adds a Magvel crossfader, beat FX and a fuller club-style control set over the DJM-250MK2. It is the closer sibling to Pioneer DJ's club mixers in feel and features.