That's exactly what i did - the frost line here in update NY is about 4 feet, so going another four feet deeper to get a full basement seemed like a good choice for me.  The basement modification to the 30x30 plan was fairly straight-forward, my structual engineer also added a staircase to the plans which connected the three floors.  I did 10" thick walls and used 3/4" threaded rod every three feet to connect the logs to the sill plate and walls.

I'm about 75% done with construction, and I'm really happy with my decision to go with the full basement.  I also added a radient heat system before pouring the basement floor as a supplemental heat source for the whole cabin. Good luck with your planning process.  Ryan