First, to address LexSchellings, Marcel Bresink's Temperature Monitor and Hardware Monitor, among other programs he's developed, are highly respected. Do not uninstall it. It is accurate and causes no problems. I have no idea why he is recommending you uninstall it.
I think you want to keep the drive well below 53C. Running the HD fan at or above 2K is OK short term, but if you keep doing that, that will probably cause premature failure. (That said, fans are a lot cheaper than drives or logic boards.) You could go higher, maybe just not as high as that. I'd aim for something in the neighborhood of 45- 47C.
I note that your screenshot shows fan speeds fror CPU, HDD and ODD along with upper and lower limits. I guess that my slightly older machine doesn't have sensors on the fans.
No, you have sensors on all your fans. That screenshot was from Hardware Monitor, which includes those while Temp Monitor (free) does not. smcFan should show you all three fan speeds. (I also have iStat Menus, which allows me to keep an eye on things all the time right from the menubar.)
Try running a fan at the back. The aluminum back of the iMac acts as a kind of heat sink. Running a fan there will help dissipate that heat more quickly and keep the hardware cooler.
Here's my setup using a $10 clip-on fan mounted to a bracket on the wall behind the computer, to avoid vibration. The fan blades are about 6" from the back, and the fan is aimed at the the upper left, in particular, but covers most of the area of the back. I turn it off and on using a power cord inline switch to avoid wearing out the cheap switch on the fan itself. Even in summer, while the back may get warm, it never gets really hot to the touch.
I periodically open the fan up, clean the blades and lubricate the front and rear shafts with a light oil.
If you scroll through this thread, I have screenshots and directions for using smcFan. Bear in mind that the default miniimum fan speeds for your computer will probably be different. Take that into account when establishing smcFan pre-sets.
https://discussions.apple.com/thread/3219643?start=0&tstart=0