Is the data so important that it is available 24/7? If not you may not need redundancy. A simple backup to a second hard drive may be all that you need. That could save you money. but if you want a seperate backup server then it's hard to top what you have there. What RAID do you plan to use?
When it comes to buying something to do a job I usually go overkill so I don't need to upgrade any time soon. I was planning on using RAID 5 but I was going to take at look at ReadyNAS X-RAID 2, I haven't had any experience with RAID before but I have read up on the subject and it seems fairly straight forward. If I understand RAID 5 correctly with this setup I will have 1 drive failure tolerance and about 6TB of usable storage and a 2-3 times increase in read speed.