If you do not have enough wires between them, you might be able to run the new wires up the wall (assuming you are not dealing with finished ceilings below the wall) by using the current thermostat wire as a pull string to get the higher conductor wire up the wall. I tried that (I ran a 7-wire and a 5-wire twisted together) but ran into a fire stop. I ended up using fish tape.
Read the installation manual for your furnace and air conditioner. My furnace was newer and was capable of running 2-stage. My air conditioner is not (and it is harder to do that on an air conditioner–but check yours). I simply had to connect the wires up to the furnace and change 1 jumper and my furnace runs dual stage. Most of the time, it is stage 1, which is 1/2 power, and that saves you in use and in utility costs. It has been below zero the last few days so stage 2 is getting its workout, too. I am comfortable. It tries stage 1 and then goes to stage 2 after a bit.
One other thing I like about this thermostat is that it will shut off the heat (burners in the furnace) or the air conditioner but keep running the fan for about 45-seconds allowing you to use that leftover heat in the heat exchanger or cooling in the evaporator that will help lower your bill a bit.
For the device handler and app, I upgraded from the basic to this one:
[RELEASE] Free Ecobee Suite, Version 1.2.
Install it on SmartThings BEFORE you install it into SmartThings.