Starting the plans on a passive home in NE Ga, from what I can tell it looks like 75 cfm will work on this 850 over 350 sip build.

Since there are no walls to run ducts it looks like it isn't simple to make the fresh/stale air ducts.

Then should I look at Erv or Hrv units can't get clear understanding for this climate.

Lastly I see a big price difference in brands, like $600-$1400

any input?

My understanding of the primary difference between the two is that an ERV also removes some of the humidity from the incoming fresh air.  It won't serve as a dehumidifier but would probably keep the AC from working as hard if that is in the plan.  If no AC, would help with the latent heat load.

I would suppose this would be preferable in NE GA, based on my living in Kennesaw for about 3 years.  Muggy as hell is my recollection (from a NWener prespective ).

Since it appears that you have a two story, duct could be run in joist space between floors depending on planned framing members.

in a hot humid climate either kind of recovery ventilator can contribute to actually increasing the latent load in the summer. Some choose to go rather with simple passive ventilation, like bathroom fans, as well as a small fresh air intake from the outside with a damper, located in the air return.

I second 3cityblue's suggestion to run ducts in the floor joist space. It is also common to create framed out chases, or use the space above a closet, et cetera. It doesn't sound like you would need an excessive amount of duct work anyway given the compact size of the space you're describing.