I am designing a 1KW DC-DC buck converter with Vin(51V battery pack with Vnom = 48V) for driving four 250W motors. Is it advisable to step down the voltage directly from 48V to 12V or should I first step it down to 24V and then to 12V? This is the first time I am designing a converter for this much power. Could you also help me with some resources to go through before the design?
I'd go straight for a single stage dual phase converter like this: -
One for each motor should do the trick.
Be warned though, this is not something to be undertaken without good tools and PCB design skills. You will certainly find more designs available if you split the power requirements by four and concentrate on a single regulator per motor load.
Stepping down in two stages will not be as power efficient and, this is to be wholly avoided if you are talking about a total power of a kilowatt or so.
Here's a good starting point - it's the selection tool I quickly used to refine a search that enabled me to select the LTC3892 above.
I've just noticed that the site I linked is playing up so here's what I searched on: -
And try this link instead.
Similar tools available from TI also.