If you are needing to replace your 10 speed shifters because they are worn out, then the cost of replacing all the other stuff you'd need to go 11 speed isn't that much more, depending on what specific group and model you go with.
Shifters, STI's in particular are typically the most pricey single item. If yours aren't worn out, ask if it's really worth it. To me, upgrades like that don't really add value to a re-sale. They might sell it quicker. But you likely won't recover any of what you spent, especially if you want me to buy it. <grin>
105 5800 is a great shifting group. I put that on my old Paramount. But IMO, if you are going to upgrade, get whatever 11 whether 5800 or 7000 or whatever you can get for the best price new. I personally wouldn't buy someone's used STI's to put on my bike. And most people selling used are asking new retail MSRP.