@Will Green once the house is moved and level your first step is to make it water tight.

If you are really worried you can go to a real estate attorney and pay them ~$300 to write up the purchase agreement with those two changes so that you know it is water tight.

As long as the building envelope (roof, walls, windows, doors) have been relatively water tight since the house was built and not allowed any mold or rot to set in, the wood will last for centuries.  

The new roof membrane may be watertight but how is it detailed where the roofing meets the parapet wall?  

Poured foundations tend to be more water tight.

The foundation can be treated with a water tight sealer that will keep water out, unless there are larger cracks or other issues to deal with.

.$2,000 New furnace after finding the heat exchanger was cracked$1,200 handyman labor to paint exterior window and trim, haul debris from yard, replace counter, get shower tile back to watertight, fix ceiling damage from water$600 materials, including paint, kitchen counter, locks$600 new vinyl kitchen floor$200 three yards of dirt to correct grading around the foundation$200 plumber to replace gas valve, fix leaky drain, replace p traps in kitchen and address a couple other concerns I had$37,050 all in.I spent about 30 hours of my own time buying materials, stopping by to manage workers, and doing some minor work myself. 

You want the place air and water tight!

It's all going to pay just to keep the door on and the roof water tight (hopefully).Let's get some more info and maybe we can figure this out.

These old windows were wrapped in aluminum but it wasn’t flat and wouldn’t be water tight.