Based on the information your load calc offers, there is very little economic incentive to go with geo since you said you have natural gas available. If you are interested in renewable, you might want to look at solar and a high end furnace and ac before you jump into geo. If you plug those numbers into the CM tool it will tell you loop length etc. You can plug in different systems and see what your operating cost savings vs different equipment is. In my AO a 2 or 2.5 ton unit would be the best fit with strategic auxiliary use, however since you intend to use your old furnace you might go with a TZ036 Or TE038 since your geo and furnace can not operate simultaneously.