Besides, you won't be able to get the core with out paying through the nose or being a manufacturer that can reasonably be expected to use it in the future.
I understand the convenience for using a Buck converter, but, for 200W+ I would think about a half bridge converter, possibly forward, as the efficiency can be higher. Transformers though, are obviously less appealing to the builder over inductors
For a 1kW Synchronous Buck converter, you're looking at a maximum of 90% (ish) efficiency, which means a maximum of 100W to disappate somewhere. Of course, 'Linear Tech' do make some of the best DC/DC converter controllers, but for such high power...its a tough project.
Also, you could parallel several MOSFET/inductors, like a 3 phase Buck converter seen on PC motherboards. The current now days can reach 30A on those....albeit with very low value inductors (400nH). I would spend the cash on a premade inductor....I'm sure there's nothing like winding your own but at that current, it'll be tough..even with big litzwire.
Coil Craft SER2914 series, use two in parallel or series.
Try BI they have about the same thing.
JW Miller 2300ht-100
Coiltronics CTX10-16-52
EPCOS Inc B82559A7103A25
Try Pulse Eng they have some nice coils.
All these above are 1/2 what you need but you could use two.