Stevehull
Yes a pre exchanger will work well but I don't believe with such a short delta T will produce the desired effect.
A local geo-installer had told me of a unit he pre exchanged for an indoor pool the help dehumidify but the ground water was in the 40-50 Deg. F range.
A car radiator is a poor excuse. Use a more substantial piece of equipment.
20x19 Water to Air Heat Exchanger Hot Water Coil Outdoor Wood Furnace | eBay
Randen