Buying a house is an anxious time, and one needs to be comfortable with all things before signing the papers. Only you can know if the house presents a value in terms of cost and associated hassles upon purchase.

Without knowing if the standing water under the house is a big pool or just small puddle(s) it is difficult to state the severity of the problem. Here are some general thoughts about the crawl space. Most crawl spaces have high humidity, generally from the lack of air circulation/ventilation.

Here are few no/low cost inspection options before spending further inspection dollars. 1) Walk around the exterior of the house and check for large visible cracks at the base of the house 2) see if the rain gutters are present and operational 3) any large trees/shrubs along the perimeter of the house 4) are there any concrete patios around the house (my guess is none)

The crawlspace dampness can be reduced by minimizing the water that seeps under the house. e.g. collect and divert roof rain water, install concrete patio (1-2′ deep) or a french drain to divert lot runoff water.

Regarding the plumbing, one of the main reasons pipes leak is due to high water pressure in the lines. You could check if the water pressure is within limits, and/or change the pressure regulator for the house. IMO most pressure regulators need replacement every 20-30 years. It is typically a couple of hundred dollar expense well worth the replacement cost in lieu of cost of replacing the house plumbing.

After your assessment if you decide to purchase the house, perhaps you can ask for a price reduction 5-10K. May be worth the effort. Good luck!