First question: Do you intend to use RedCode 28, 36 or 42?
For argument's sake, let's assume Redcode 28. That is by far the easiest to handle, since it means only 28 MB/s or 224 Mb/s, almost ten times what HDV or AVCHD require. One hour of recording translates roughly into slightly more than 100 GB.
Editing this kind of material can be called 'data-pumping' to the extreme. I'm not sure what RedCine-X does to those figures but it is safe to assume it will still be a very hefty data stream, even after conversion.
Even though you said 'low budget', this kind of editing may require an i7-980X with 24 GB memory and a GTX 480 as the bare minimum. But the disk setup? These are sizable data streams to handle, so the faster your disk system is, the better. Just two RE3 disks in raid0 are not enough. For these amounts of data, even if they are not taxing on the CPU, you need a very fast disk system. The problem is that SSD's are way too costly with these massive amounts of storage, so they are out. Conventional disks are the only sensible and affordable solution, but you have to use a good raid controller and large numbers of disks.
Without delving too deep into Red specific aspects, my gut feeling is to get an Areca ARC-1880iX-12, 16 or even 24, populate it with 4 GB ECC cache memory, add a BBM (battery backup module) plus a good UPS and then select your drives.
Even with these large storage requirements, I would go for 1 TB disks and not for 1.5 or 2 TB models because of the premium price. You will benefit more from a performance point of view when you use 12 x 1 TB than 6 x 2 TB, and the price will be more attractive too.
Given the 'low budget' starting point and your remark about limiting the number of disks, I would seriously consider the very fast Samsung F3's. Every extra $ 55 you can spend on an additional disk will improve your performance and storage capacity. Your idea of using the existing 320 GB disk for backup makes sense for your projects, but for media and the large amounts of data it is too small, only three hours of raw material, so you may have to investigate a large iSCSI NAS connected with dual NIC's .
Just my initial thoughts.
No you do not want to put the OS on the raid 0 that will handle the video files especially when dealing with R3D. You can get away with 3 drives in your system if you edit 1 or 2 layers at a time dealing with this material. However the caveat is you will likely have to render the timeline for clean playback more often than not. If that workflow is good enough for you to learn the Ropes with R3D in 4K then that will work. I however would recommend upgrading the drives to 2x 2 drive raid zero arrays for video asap when you do have the funds. That 950 will also limit you editing those files. If time is not a factor though you can workflow through that.