You don't need that much. All three alternatives would require approximately the same PC/Server and raid controller. Only thing that might need a decent machine is if you decide to run TrueCrypt or similar, to encrypt all disks (as I have). This require some CPU power.
Pretty much any machine can run this, as long as it's a real Hardware raid controller that does all the processing. And to take advantage of the throughput of this units, you'll have to use Raid Cards for at least 8x PCI-E buss.
To connect the unit and raid card you'll need one Mini-SAS SFF 8088 to 8088 cable, external to external. IF you purchase a card with internal connections you might need two cables an a converter from internal (SFF-8087) to external (SFF-8088). Or a single cable that have connections from internal to external. Not sure about this. But when you go for this solutions the best thing would be to get external raid controller connections anyway!
As I mentioned, I have a HighPoint RocketRAID 4320 with two internal Mini-SAS ports my self (this one is a few years old, so you might want to check newer models). This is supposed to be a rather good card, and it have worked in all my MS 2003 and 2008 servers I have set up. I have not tried it on linux or anything yet, but I think those are supported too. If you like, you could go for a cheaper card with 1 Mini-SAS connection, but this would also mean that you can not just add another box and get the same throughput. There will be some limitations with for example 48 disks vs 24 disks on one SAS cable. Not sure how much impact though. It's up to the controller, what it can take I think. As far as the Cable and DS-24E units go, I think they can throughput up to 2400MB/s, so you should be able to daisy chain at least two expanders on one port without suffering to much. The HighPoint RocketRAID 4310 as an example have a limit at 800 MB/s on one port. So this is where it will struggle. Two ports and two boxes is twice as good. So, you'll gain in performance if you expand one day.
The important thing to remember when purchasing a Raid Controller card is:
- How fast a PCI-E slot is required. You should aim at 8x or 16x
- If its a REAL hardware raid controller
- Supported Raid types (you should require Raid 5 or 6 for storage space, speed and safety)
- How many ports and what type (E-Sata or mini-SAS)
- OS support and disk support
The same things apply for a E-SATA raid card as well. Only difference is that you need much more ports. So the price difference in this field is not much. One thing you CAN do to lower the cost is to go for a software raid controller, but I doubt that you'll get software raid controllers with mini-SAS ports. And it will reduce the speed of the unit. Significantly I think. So, it's a no no imo.
Other than that, I would search for different models and check out reviews for this cards. Make sure all of my points above if checked, and then compare it against other models for price and speed for your desired Raid setup (1,5,6,10 etc).
Two quick suggestions based on my experience, from Highpoint:
HighPoint RocketRAID 4322 (2 external mini-SAS ports) - 350$ +
HighPoint RocketRAID 4310 (1 external mini-SAS ports, same chip, but half the RAM?) - 280$+
And a few comments about 15 disks in JBOD; are you mad?
If ONE of your disks fail, your entire disk setup with be destroyed. All your data. JBOD is used with a very few disks, of it's even used at all. It gives you poor performance and it's 15 times LESS secure than a single disk. At least consider using Raid 5. This will only use a total of 1/15 of your disks for parity data, and it can reconstruct all your data if one disk fails. If you don't need the security, then set it up as a Raid 0 for a lot better performance
What's good with the 24 bay disk expanders is that you have so much space that you can make several arrays, and do what you like with them. You could add one array of 12 disks with raid 6 for great speed and great security, and you could use the other disks in Raid 5 with great speed and good security. It would act like a double backup really, while you know you get the speed. And the automatic backups would go very fast....
I don't know what your existing cabinet can do. But JBOD is scary. Even for backups. I've never used it. One of your disks is bound to go to hell one day or another.