we are looking into geothermal for heat and hot water but are building in a sandy soil in the far northern suburbs of Minneapolis. Are  the sandy soil conditions good enough for a closed ground loop or even a slinky type of installation and which would you suggest?

You should definitely look into DX geothermal by EarthLinked Technologies. See link With sandy, rocky, gravel/cobble, wet, dry, clay, silt, etc., it doesn't matter. You just need to know the temperature of the earth. That's usually well known for your area, so you don't have loop design guessing going on, and you can rest assured you get the most efficient closed loop system out there.

If your soil is sandy (completely rock/pebble free) down to 5-8 feet deep, I'd do a horizontal copper loop (for DX geothermal).