The cheapest, simplest and easiest way to switch to a double is to unbolt the granny ring from your current crank and reset the low limit screw on your front derailleur to lock out that position.

Otherwise you are in for a major expense to what benefit? The 5800 group will require a bb tool to remove your square taper bottom bracket cartridge and a crank puller to remove the current crank, a different bb tool to install the Shimano external bearing bottom bracket, and a new 11-speed rear wheel.