Well, that doesn’t sound too demanding, so on the inexpensive side I’d say, a RAID with drive running at least 7200 RPM, stay with hard disks and wait until SSD’s come down in price and up in reliability some more. I’ve heard horror stories from friends who have had SSD drives failing in only a year’s worth of use.
I don’t know what interfaces your macbook can take advantage of, but everything today is trending to Thunderbolt, so a thunderbolt-based RAID and any adapter needed to interface with the fastest port on your macbook would be my starting point. This way when you eventually upgrade the old macbook, the drive is still useful with the new gear.
Get the biggest RAID you can afford because there is no such thing as having too much storage. Particularly when one big project is still hung up, waiting on approvals, but you need to start the next big one.
If you were doing stuff for broadcast, the suggestions would be different and there would be more stuff involved like external monitors and the like. FCPX is designed around a 64 bit processor so older macs won’t see a performance boost like a new one will.