Tobi, Bob doesn't answer questions here, so I'll offer an opinion for what it's worth. Don't consider this an optional fix. Work out some kind of price and approach (which should include abating the current mold/mildew) with the current owner, perhaps adjust your offer, and make the fix a condition of sale.
As far as what the fix is, that depends to some extent on the source of the water; is it from rain or a high water table, but whatever way you go the approach should include covering the dirt with polyethelene sheeting (Tu-Tuff is best) run up the sides of the foundation, with all seams and edges taped. In fact, unless there is visible wettness, this alone may be sufficient.
The stated 25% heat loss due to lack of insulation sounds like a rather gross exageration, especially since there are no vents. The largest heat loss tends to be from warm inside air exiting through the attic. This in turn pulls in cold replacement air, often from the crawlspace, which makes the floor feel cold. You may want to add insulation there eventually, but always start from the top and work down.
As new homeowners to be, the best thing you can do is start to learn how a house works (or doesn't) so that you can make informed decisions. A good place to start is at www.weatherization.com. You might then want to read through the "Air Infiltration" and "Moisture Mold and Mitigation" threads on This Old House BB (www.pathfinder.com/TOH). Good luck.