You are right in replacing you HDD with a SSD, it will give your iMac a new life! I'm personaly using a samsung 840 pro on my 13" mbp 2012.
I don't know about a braket in order to hold your 2,5" SSD in place of your 3,5" mechanical hard drive. What I would recommend is to keep the actual HDD in its position for the data storage and use the optical drive bay for the SSD. Doing that, you skip the problem with the thermal sensor 2,5/3,5" braket. Optical drive will need to be removed but it is not too used these days, don't you think?
Check out this iMac & Mac mini Dual Drive Enclosure for that purpose.
You can install your system on the SSD, data on the HDD. You can keep your optical drive in a drawer and eventually buy an external adapter in case you need to use it one day.
For the SSD to buy, you should read this already replied Which SSD should I use to replace my HDD?
Make sure to take into consideration all these AHCI, TRIM stuff. You need the TRIM to be active in order to keep respectable performances with your SSD. Please ask if you need more info on that, there is probably some one that can help. You can refer to this article on How to Enable TRIM on Your Mac's Solid-State Drive