yes, a 2013 Nexus 7 will automatically prompt you to upgrade to Marshmallow
I find SkyDemon works very well on Nexus 7, and personally I find it a preferable size for wearing as a knee board than the iPad 
I've run SkyDemon on a Nexus 7 for around 2 years, and other than a few wobbles when Lollipop first came out, all's been good
remember, unlike some devices, Nexus 7 has built in GPS, so no messing around trying to attach an external GPS either.
 

I've been very pleased with my Nexus. It has lived up to all my expectations of it. If you do go ahead an buy a Nexus, it probably won't have Android 6.0 (Marshmallow) installed initially, but with a few days, you will be automatically prompted to upgrade.

Mike
_________________________________________

Many thanks - all appears to be working fine with Marshmallow.
Also would concur that SD works well with the new Nexus 7 which does have a very good bright screen.