Sledboy,
If I may add -- usually a mid-winter survey is no big deal unless we're in the midst of the kind of freeze in which we find ourselves now where below zero temperatures have been the norm.
Any H2O intrusion will have frozen thereby negating any accurate soundings, and may also show false positives in those areas that may have such H2O intrusion but otherwise percussion sound out OK.
Therefore, as Al stated, be very careful with the manner in which the integrity of the hull, deck, and attendant stringer(s) systems are tested.
If the seller is willing, you may be able to come to a $$$ agreement now, but 'put-off' the survey per se until the temperatures rise dramatically, conceivably still a couple of months away. In the alternative, can this boat be moved into a heated building for a few days to warm-up in order to facilitate conducting the survey now?
I see no useful purpose in trying to heat the bilge in the kind of cold conditions that we are experiencing.