I've done many hardware upgrades and drive swaps. But I have the same model listed (2.6 GHz quad i7, 16 GB RAM) but I also have dual 1 TB spinners in the box, and I am terrified of opening this one and trying to swap the spinners for an SSD. (The only upgrade I did was to max out the RAM to 16 GB and that was trivial.)
I bought an internal SSD (500 GB Evo 850) but got cold feet before attempting to install it -- I instead got a SATA to USB 3.0 cable and hooked it up externally and I boot from the SSD connected externally. I think I lose a little throughput but not much -- I can still sustain something like 440 MB/s on most reads and writes, instead of around 530 as rated for internal. Good enough for me, considering this model is pretty much like major surgery to swap a drive. Maybe some day when this is a secondary or tertiary Mac for me, and the resale value is almost nil, I might chance it. Not now.
As I own this model (but with 2x1 TB HDDs, not an SSD) I've watched the market value of these and if it works well, $460 is a pretty good price. If you wanted more SSD storage, I'd simply keep the OS and most heavily used apps on the internal SSD and buy an external SSD for more storage. In fact, depending on what you are running, 256 GB of SSD might be enough for you (with the OS and a few important apps on it, leaving at least 50 GB or so free) and many other apps, music, photos/videos and other stuff would be fine on an external USB 3.0 HDD. The OS and some apps benefit a LOT from being on an SSD. Some other stuff, not so much. For example, I use VMware Fusion (on the SSD) together with the Mac OS (excuse me, macOS) on the SSD and a virtual Win 10 disk on an internal HDD and it works almost as well as if I put the VM file on the SSD itself -- in fact, it's much faster than a NATIVE Boot Camp install of Win 10 on HDD. As I didn't want to burn 100 GB of my SSD space I left it on the spinner -- having the host OS and Fusion app (about 700 MB) on the SSD was good enough for most of the reasons I have to occasionally use Windows -- a few apps I need that don't have a Mac version.