We are buying  house built in 1904.  It's large (4000 sq.ft) and has a carriage house (900 sq.ft.) which we will rent.  The house needs everything -- from plumbing to electrical.  There is currently a well on site that we will not be using (we intend to hook in to the municipal water supply).  Can that existing well be used for a geothermal system?  If we were to go to geothermal, on average, how much more is the up front cost?  This house is in southeastern CT.  Are there any issues with respect to that climate?

We are also investigating switching the power source over to solar since there is a very large garage that may accomodate panels fairly well.

The bottom line is:  I am interested in rehabbing this old house and making it as green as possible.  I would like input as to where to begin.

do you know how deep the well is?sure its open all the way? you can probrably use it one way or another,if your doing a closed loop system you'll need several more boreholes drilled.to handle the load of a old 4000 ft house.If you went open loop standing column you would likely need to drill down to about 1500ft. both are very viable and common in your area . 

Lots of un answered questions (regarding required btu's etc.). However to help with one question you had, a geo system (particularly open loop), costs little more than a fossil system if you are contracting the duct work at the same time.
My concern would be well capacity. You sound like you may require multiple systems which couldmean 2 million gal/yr of water required.
You also need to drain this water somewhere.
I also do not think you would find operating multiple geo systems a simple task for solar.
Good luck,
Joe
Joe

First step is a detailed heat load calculation. Given the age of the house, insulation, infiltration and windows are are likely major issues / areas needing major improvement.

Completely greening a 4000 SF 1904 house could run deep into 6 figures.