My problem is that the townhome sits over a lake, and even though I got the insulation updated when I got new floors, apparently it wasn't good enough as the moisture underneath the home has damaged the subfloor.

More often than not these basements get a high amount of moisture.

All leaks had significant moisture enter the property at ceiling and wall.  

If your budget allows a maximum of $250,000, do not consider coastal California.Climate - Properties in areas with hard freezes and excessive moisture will tend to require more maintenance than in milder and dryer climates.Property age - The older the property, the more maintenance it will require, unless all the major systems and components have been updated recently.Taxes - Both income tax and property taxes are a direct hit on profitability.

It grows primarily in shady places with adequate moisture.

I made a full price offer on a bank owned property, and the only thing I asked is for is a home inspection, and moisture/termite inspection at my expense.

You do not want duplex down without permits can cause major structural issues, you need dig down, safely cut big enough windows and foundation/walls usually need work to not let moisture in. 

Roof seems to be in its last days and causing some moisture intrusion.

To keep the moisture in, you can spray concrete sealant on it.

The presence of trees near a building can also destabilise its foundation and drains.Cracks on the walls: Variations in moisture and temperature cause frequent expansion and shrinkage of the concrete, causing cracks.