At that price you can get yourself a really nice Raidcard. If you want hot swappable drives you can look at some drive bay adapters that convert 3 5.25" to 4 3.5" drive bays. I think Raidsonic makes some that are hot swappable. (called Icy Box something) Just to give you some ideas on a case, the Xigmatek Utgard would be a good idea, since it is cheep and cheerful and offers a lot of drive bays. There might be some fulltowers having more. For motherboard and CPU I would go with something like an H67 or even H55 bundled up with a Core i3 or low end Core i5. You could use the onboard graphics, so that would save you some money, which could invest in a better raid card. Just did a quick scan and here is a 28 port raid card for around 1000 pounds.
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I'm not the expert when it comes to raid card, but that one does look really nice
EDIT: Oh and some of the big Lian Li cases have 12 5.25" drive bays, which would be able to convert to 16 hot swappable 3.25" drive bays, I would think.