Last June I bought my second Refurbished MM from the Apple Store which is a MM(Late 2014), 2.8GHZ,8GB RAM, 256 SSD for around $850.00 plus another $100.00 for Applecare and around $60 sales tax. It is one great machine AND as you are probably aware Apple's policy on their Refurbish Machines is like buying a brand new machine in which a "new one" would have cost over $1000 with the above "specs" not including the cost of the AppleCare and State Sales Tax.
My other Refurbished i5 MM(Late 2012), 2.5GHZ,4GB RAM, 500GB HDD which I bought in August, 2013 and in which I upgraded the RAM to 16GB last January. This is a very good machine, however, my MM(Late 2014) runs circles around this machine due to the much slower HDD. Adding the extra Ram did not seem to increase the speed very much. I now use this MM as a Media Server. I plan to add a internal SSD when my AppleCare expires on this machine next year.
Of note, on this MM I am still on OS X "Mountain Lion" and I have no plans to upgrade the System Software until I add a SSD to this machine.
I would recommend a MM with a SSD as I have no experience with a Fusion Drive.