I would agree it is a good deal. Without knowing anymore than the length of slinky tube in the ground the 1200 ft is between 1-2 ton. I see two options 1/ use the 4 ton compressor with Propylene Glycol in the loop and a cold limit control like what is used in a refridge. to limit the GSHP from turning the ground to permafrost (block of ice in the ground). This is a double edge sword on one hand there is a lot of heat avalible as the water in the soil changes state from liquid to solid the downside once the ground is frozen you are finished heaating for the season. As well this last bit of heat is expensive to obtain as you COP has dropped dramatically.
A side note: I had been told of a sizable installation in a high rise area inwhich a small loop was installed with a sizable heat pump. Being the loop was always in a shadow and covered with snow and buried deep the ground froze and couldn't recover because of receiving no sun.
2/ Change to the smaller compressor utilizing the larger HX and still install propylene glyco and monitor your loop temp. Remember these things are suppose to run almost continuosly as the heat slowly migrates into your loop. With the smaller compressor you will enjoy lower operating costs. Your super insulation work and being you won't need 75 Deg F in your garage you might sneek by.
Randen
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