For SLOG (ZIL) use, you want a device with supercapacitor or equivalent in order to assure that the device is able to actually commit to flash the things that have been sent to it. Intel's 320 line is the cheapest suitable thing I've been able to find for that, and, bonus, it uses an array of capacitors instead of a supercap. However, it is MLC. So you underprovision and let wear leveling do its thing.
Mirrored SLOG is unnecessary with ZFS v28. If the SLOG device fails, the system will revert to using the on-disk ZIL.
Splitting an SSD for combined SLOG and L2ARC use is not supported under FreeNAS, though you can probably hatchet it in from the command line. Still, since the cost per gig of an appropriate SLOG device is much higher than the cost per gig of an appropriate L2ARC device, the smart move is to purchase a small SLOG device and a much larger inexpensive L2ARC.
The 840 would probably be a fine drive for L2ARC. Be aware that the data structures to support L2ARC are stored in ARC, so ultimately it is suggested that if your server can handle up to 32GB, you are better off upgrading RAM to 32GB and then adding L2ARC on top of that. Otherwise, the addition of a large L2ARC device will actually dramatically decrease the amount of ARC available. On a 16GB system, by default, that's only going to be about 9GB to start with. Each entry in L2ARC eats 200 bytes of ARC. A fairly conservative suggestion is that L2ARC can be about 10x the size of ARC, and aggressively perhaps as much as 20x the size of ARC. Many factors play into how successful any given configuration is, though.
You can probably attach a 120GB SSD for L2ARC without a problem with your existing 16GB of RAM.