Help me design a home server for Photoshop work. Reliable, easy to expand, and cheap are the priorities. Lots of sub-questions inside. I want to build a home server for heavy Photoshop use (1 GB+ file size).

The goals:
- A system with a few TB of storage that is easy to expand in the future
- Reliability is #1 concern - must tolerate a disk failure
- the user will re-save files at all major stopping points, so write performance must be decent
- Power consumption and noise are not important
- The server must run a CrashPlan client to sync all data to the cloud
- I would like for all the storage to appear as one large volume, even after adding new disks
- I will be the tech support, so I need remote access via the internet
- Money is short, $500-800 budget


My tentative plan... Does this sound good?
- Headless Linux box
- One Logical volume of concatenated RAID 1 mirror pairs
- 8+ drive bays, preferably 12, add new mirror pairs as needed
- mdadm software RAID - I've read it's as fast as Intel motherboard RAID


Many questions:
- How do I get lots of drives? Most motherboards seem to have about 6 SATA connectors.
- Should I go with a rack or tower case?
- Is there a way to cache network writes to the server memory, for increased apparent speed to the user?
- Are there any pitfalls to constantly running an SSH server for remote access? How do I deal with the dynamic IP?
- How do I make the server send an alert email when it detects disk failures?
- What must I do to make the server restart everything automatically if it loses power? Is a shell script enough?
- Is there a way to make the Photoshop machine's local disk look like a cache for the server, so writes go to the local disk immediately and are then moved to the server with something like rsync?


I know this is a bulky question... thanks for answers to any part!