A USB 1.1 cradle will not work with a 250GB drive -- well, it will work, but you'll only be able to use 127GB of the drive, due to limitations of the USB 1.1 standard. You'll need a USB 2.0 cradle.
Phatnoise Media Manager will NOT work with a 250GB drive -- at least no one on this forum has ever reported success in that regard. There may well be a hack to make it work, but it remains undiscovered. (Windows Vista also seems not to like that drive.)
Phatnoise Music Manager will work, to some degree. VorTechS' Phathack Media Manager will work with the drive.
If you want hassle-free operation, personally I recommend going with a 160GB drive.
If you are going to use your Phatbox with a Kenwood radio, you will need the Kenwood cable to connect it.