Could I replace the internal HDD with something much larger?

First I should warn you the newer 'Thin series' iMacs are a bear to open! We've had a few people here kill their displays in the process (review some of the Q's posted here) which is an expensive oops! You will need the correct tools and you really need to follow the guide to the letter as its real easy to kill the display (I've killed one!) And we thought we knew what to do (to be fair it was before the guide was completed).

The second point here is the system is designed to run with minimum amount of fan noise. So when you alter the internals with a hotter running drive you'll start to have more fan noise as the system combats the added heat.

As these larger drives are so new I would hold off a bit. You also may find you really don't need that much internal storage. External RAID drives can offer equal if not faster response.

OK, so you still want to do this! Follow the IFIXIT guide here: iMac Intel 27" Retina 5K Display Hard Drive Replacement. You'll also need this cable: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later as the drive you are putting in likely doesn't have the required signaling for the SMC services to monitor the drives temp to scale up the fan and besides having a second set of tapes will be helpful.

As to the doing this you'll need to make a backup of your data. You'll need to break the Fusion drive set so the two drives are seen discreetly on the desktop. Only then can you take the drive out.

I do recommend you prep up the drive externally beforehand. You only need to format it using Disk Utility so its GUID. You'll need access to the internet for the system to go into internet recovery.