I have a Harmon Mark II in my finished basement and it works very well. I live in a 3,500 sq ft. log home that really isn't laid off well to just heat with one coal stove. ( I also have a coal boiler in my garage to heat the house) I usually only use the harmon when it is'nt really freezing outside. Once it gets really cold I fire up the boiler.
You will be able to heat your basement for sure with a Mark III but it all depends on the layout of your house and how tight it is. If it is insulated well you will be suprised at the heat outout !!
Either way, It will atleast cut down on your heat bill and pay itself back in no time.