There is a $500 rebate I will miss out on if the duct work is in the ground. Bottom of this article.
https://www.2escore.com/Documents/HVAC_1114_v6.pdf
Might just be better to go the the attic space. It will be in the envelope anyway since the house will have a sip roof. Though I really like the warm floors idea of them in the floor. Total duct work in the floor looks like close to $1,500 for pipe and fitting.