I have been doing some research on the 840 Pro for a few weeks now as I am also interested in switching to the SSD community. There are a few websites that show the all the specs of the 840 pro. I ordered the 840 pro 512Gb for my macbook pro and plan on adding another one later on. I would like to set it up into a RAID configuration. Youtube has some great videos on how to do this if you are interested.
If you can afford it, get the larger drive. While benchmark performance of 256gb SSDs is usually a bit higher than the 128gb model, you won't really notice it in every day use. The real reason to get the larger drive is to have more space to use for programs and data.
As far as the cable issue, you will have to see when the product information is posted, but the drives that I buy never come with cables, maybe screws or drive caddies but not cables. Fortunately, I have a whole drawer full from all the motherboards that I buy.
I'd also buy the 256GB SSD. Gives you more room to put your programs on it if you want, in addition to a slight speed advantage.
The 830s came 4 ways, the simple bare desktop SSD, the desktop conversion kit with screws, adapter plate, data cable, the plain laptop SSD and a kit with extra components if needed. Possibly there are similar packages for the 840. Just to be aware of. Sort of the same thing where you can choose a bare OEM HDD, or a retail version with the instructions, screws, cables etc.