I wanted to see what it would cost to have a provider upgrade the hard drive. People that have the skills can do this for a fraction of the cost.
1TB HDD - $76.99 plus tax.
or
500GB SSD - $118.99 plus tax for the drive and $21.99 for the appropriate mounting bracket.
Labor - $149 for either option
I would not put a HDD back into this computer. You are just asking for problem by doing that (GPU).
Compare the prices to doing it yourself:
512GB SSD - $46
3.5" to 25" SATA Adapter - $7
Buy Team Group T-FORCE VULCAN G 2.5" 1TB SATA III 3D NAND Internal Solid State Drive (SSD) T253TG001T3C301 with fast shipping and top-rated customer service. Once you know, you Newegg!
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If so, the ORICO 2.5" to 3.5" Hard Drive Adapter is perfect for you. It turns your 2.5-inch HDD or SSD into a 3.5-inch size. Compatible with Windows, Mac OS, Linux Desktops Supports all 7mm / 9.5mm / 12.5mm 2.5" SATA hard drives, including HDD and SSD. Supports Serial ATA I / II / III. ...
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If you decided to get a 1TB SSD - $80
Buy Team Group GX2 2.5" 512GB SATA III 3D NAND TLC Internal Solid State Drive (SSD) T253X2512G0C101 with fast shipping and top-rated customer service. Once you know, you Newegg!
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You probably don't have the tool set to do the job, here is a kit from OWC for $47, it includes the external temp sensor SATA adapter-
OWC DIYIMACHDD09 Improve Performance and Capacity. Upgrade Your iMac with Confidence. OWC's in-line solution is the only hardware digital thermal sensor on the market allowing a third-party SATA drive to be installed in your iMac. A convenient, plug-and-play cable, the In-line Digital Thermal Se...
www.amazon.com
Of course I would expect a warranty with having a repair shop to do the work, so that would have to be considered.
People that have the skills
I wonder if they will include an external temp sensor? I would ask prior, as I am pretty sure your iMac will need one when replacing the HDD with anything third-party. The SATA Temp sensor cable itself is really easy to install, doesn't require any skill and it is done when connecting the new drive.
But, I wonder if the repair you talked to knew to install one, as not all Macs need one.
The reason why there needs to be an external sensor is that Apple's OEM HDDs for your Mac have a built-in temperature sensor. If the HDD is replaced with non-Apple drive, it causes the OS to think that there is something wrong with the sensor, and protects the HW by running the fans on full blast.
This can be easily fixed by putting an adapter between the SATA connection going to the SSD and the SSD itself. Most external sensor adapters have adhesive on it to stick on the SSD. There are other HW solutions that are basically free, like shorting the pins fo the temp sensor for example. This would read as the temperature around the drive is cold all the time.
There are also SW solutions, like Mac Fan Control, but I figure that the external sensor isn't that expensive, might as well use it and forget about it once it is installed.
The only reason I bring it up is that the external temp sensor wasn't included in the quote you posted to us. With the "professional" install one?
I am pretty sure your iMac needs one, so it would be a good question to ask.