mac mini had 2 refreshes in 2009, early and late
once in mid 2010,
once in mid 2011,
once in late 2012,
once in late 2014.
Going by the pattern, the next one should be late 2016.
For 2014, ram cannot be upgraded. need to upgrade from day 1 with apple. hdd can be replaced with ssd but the 2nd bay will not be able to use. recommend to buy the ssd option and then add the dual hdd kit yourself. alternatively buy the fusion drive to get both cables.
For those reasons, the 2012, especially the i7 models can still fetch very high price... 2012 machines can upgrade rams and hdd / ssd on your own easily...