Hi Frank,
ProRes is actually a preferred format for PBP, I usually stay away from H.264 whenever possible.
Just make sure the machine can handle the processing, and that the drives can handle the data rate, most likely a RAID 0/5.
As for the person telling you to keep the file under a certain size, they have likely never dealt with 5K.
With ProRes, you might be looking at 10 Gigs+ depending on the frame rate.
Vince Becquiot
Indigo Live | Kaptis Media
San Francisco Bay Area