The 2018 is a complete redesign. MB hasn't released any info on it yet, and none of the upfitters will have any info, either.
The only thing we know about the new Sprinter is that it's going to be FWD with an interior more akin to the Metris. Aside from that, it's all speculation and rumors.
The 2017 is the same as the 2016 with the only difference being the addition of a leather wrapped steering wheel and swivel seats for both driver and passenger.
The Espar D2 is purely an air heater. You can use a D5 for hot water, heat, and engine pre-heat (sort of), but you won't find this setup on Espar's website as they just make the heater itself. You need all the air handlers, flat plate exchanger for hot water, etc. Rixen is who makes the kit that SMB uses.
This explains it roughly, and the link at the bottom has the itemized parts list.
Sprinter and Class B & C RV’s
It'll be easier for us all to provide feedback if you give us some details about yourself, what you're going to use this for, and any other goals you might have.