I would save your money on the i7 chip upgrade i have both an i5 and an i7 imac both with 16gB ram and unless you are rendering video for hours you will not notice any speed difference whatsoever. I would spen the money on a bigger SSD which made a night and day difference to my machine. If you dont fit the HDD however you will have fan problems and as the cost of the OWC temp gizmo is about the same price as a 1TB HDD i would fit that as well.
I dont know if the 2010 model has the "extra" sata connector that the 2011 model has? If it does you can fit the HDD and the SSD ,(and keep the optical drive) reconnect the temp cable and have no fan problems. Apple fitted three different HDD, Western Digital (my favourite) Seagate and Hitachi , they all use the SMART protocol which OSX uses to measure the internal temp of the HDD . The only problem is that all the manufacturers use different pins for the little two wire plug that goes into the logic board , do you have the lead? Also the lead is fitted with different two pin plugs for the different manufacturers, I have always fitted Western Digital drives and the temp sensor goes on the two pins farthest from the sata connector on the top row of pins. Connect that to the HDD temp socket on the logic board and you wont have any fan problems. I have also in the past Squeezed a Hitachi plug onto a WD drive (the pin spacing isnt quiet the same) and it works . Fortunately I have a draw full of stripped down old Video cameras et al which are a continuing source of plugs and wire for all uses.
OK your photos show the SMART cable which is the 4 pin one , I could not see which pins the two wires were on but it looks like a Western Digital plug , there are 8 pins , two rows of 4 on the WD drives , the connector goes on with the two wires on the top row of pins (with the drive installed) the furthest from the SATA cable , the other two connectors you show are the SATA data cable (small one) and the power cable for the HDD. I dont know what you mean by the other OWC kit. I dont use OWC kits . All you need is a sata data cable and a power splitter cable which you fit on the hard drive power connector , one of the splitters goes onto the HDD and the other goes onto the SSD , the ones I use actually have two spare ways , I just cut off the one I did not use. You then put the sata cable onto the SSD and the other end , which is a right angle sata onto the spare sata socket which is on the inside of the logic board where the Optical drive plugs in. You can then tuck your SSD away under the video card heatsink. If you follow the OWC video for fitting their kit its the same, you just dont have to spend the inflated charges they charge ,my leads cost £5 from ebay here are the links
The connection on the logic board is just two pins and is the same on all HDD cables . The four pin socket should have two pins at one side connected and the others blank , just put it on the new drive with the wires farthest from the sata cable , its fairly obvious once you get the HDD where they go. aThe OWC kits are a bit of a rip off , you don't need the carrier ( which is just a bit of plastic ) as you are not fitting the SSD in place of the HDD but along side it . In reality you do not need anything to attach the SSD it is a snug fit behind the heatsink riser , in fact you need to make sure your cables are tucked away neatly to stop them pressing on the LCD cable when you replace it .You could if you want put it behind your ssd like in this video
Unfortunately "Hens teeth" springs to mine , there are a couple on ebay at rip off prices £15
As I think I mentioned elsewhere I have both an i5 and i7 IMac of the same generation and can't tell them apart as I don't do anything that tests their computing power to any great extent , except the i7 version tends to run slightly hotter .