I agree fully with SpecFoto, a new unit will last over 10 year, will run faster and cooler and includes a warranty.Lucky this is the Late 2015 model which has the Retina screen and a Fusion drive that has, most likely, a 128GB SSD blade with the 2TB HDD, but it has Thunderbolt 2, and not 3 which is more compatabile with the newest USB-C devices. The Mid 2015 did not have a Retina display or a fusion drive, hence no SSD and you want to avoid the mid-2015 for this reason. The SSD makes the Mac a lot more responsive for startup and opening apps. But the HDD makes the iMac a lot noiser, run hotter and you can't believe the dust that accumulates inside. The iMac pulls air from thin slots at the bottom of the screen and out the back vent, but of course dust settles on the top of the vents and can clog them up pretty bad. My friends 2012 27" iMac come to find had about 1" of dust in the bottom and ran so hot it fried the HDD prematurely. Since you want to keep this iMac I would eventually replace the 2TB drive with a 2TB or larger SSD, I did this with 2 different 27" iMacs, it is not that a hard to do.
But as an alternative here is link to a new 2020 27" iMac with 6 core I5 processor, 8GB of memory (can be expanded very easily), a 4GB graphics card, a 256GB SSD blade and 4 USB 3 and 2 TB3/USB-C slots. It's only $1649 and for sure will run faster, quieter and cooler than the 2015 model and you get a 1 year warranty (more if you buy AppleCare). This is a perfect iMac for what you listed you want to do with it. If you have large users files for about a $100 you can get a 4 or 5 TB My Passport WD portable drive that runs pretty quick thru the USB ports and you don't have to worry about the heat and noise. I use these for my iMac, actually 2 sets of 2, set up as a 10TB RAID and 10TB BU RAID where my 7 TB of users files are stored.