What Size Drive for Media Cache?

Hi there,

I'm about to edit a large project (5TB of 4K RED footage and counting) on a laptop and am intending to use the setup below for disks. Ideally I would have gone for a "4 disk" setup, but I've maxed out my eSATA connections and would have to use USB for anything else. If coincidentally you can see any problems with this configuration please let me know!

Disk Setup

1. C Drive 750GB (OS, Programs)
2. 3x 2TB Drives in eSATA docking stations (Media, Projects)
3. Second internal hard disk (Previews, Media Cache, Exports)

I'm intending to install a second hard drive in my laptop to use for the  Media Cache. What size/speed drive should be used for this purpose?

Thanks!