What you're looking for is a bit odd. Direct attached storage devices (DASD), like SAS or ESATA enclosures are pretty common. When you talk about "replication over the network", though, you cross a boundary into something more like SAN or NAS devices.
Block-level replication of DASD could be done via software on the host OS. You don't mention your OS, but DRBD would do what you're talking about on Linux. The storage devices aren't doing the replication, though-- it's the host OS. (I'm sure there's a DRBD-alike for Windows, but I don't know of one off the top of my head. There are some rather pricey looking offerings like SteelEye Data Replication and Double-Take Availability that will probably do what you'd want on Windows.)
I suppose it would be possible to build a DASD device that contained an integrated controller to do block-level replication via a network. Since there wouldn't be contention for filesystem access with the host OS this would be feasible, but I've never heard of such a device.
File-level replication of DASD could be accomplished pretty easily by the host OS, using something like rsync, Unison, or Microsoft's DFS replication (as well as a whole host of third-party solutions).
Having a controller in the storage device itself perform file-level replication would be fairly difficult since it would have to contend for filesystem access with the host operating system. (Presumably you could use a cluster filesystem and skirt that issue, but cluster filesystems aren't commonly used on commodity operating systems.)
I would think your best bet would be some cheap DASD and a host-based replication system.
We just did it. You need just 2 parts at a cost of around 100 EUR/USD: the backplane (511787-001) and the cage (511784-001). Additional cables were not needed.
Some sites say that the SFF cage can be converted to LFF by removing a metallic inset, but this is not true. The SFF cage included with the server cannot be altered. Maybe in the past the conversion was possible, but at present it isn't.
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Impossible to say without knowing the model and how the device is connected to the server. DAS is a very loosely used term.
That depends on how the DAS is connected to which device. For this to work, you must connect the DAS to the ports of the hardware RAID controller and it must identify the single disks separately. If that is not possible, you can't use the disks in a hardware RAID.