Once you are in the chain of title, even for one minute, you are 100% liable for the cost of cleaning-up soil and/or groundwater contamination unless you had a proper Phase I performed.

Also, there is a ground water contamination addendum attach lets me know it is bad I had to sign off on?

Typical issues with basements relate to gutters failing, grading being toward the structure or flat (surface water doesn't flow away from the building), grading funneling offsite water to the building, or ground water being high enough that it is against the foundation.

Of course right about the time we listed the area we were in was listed as a toxic area that had elements in the ground water causing cancer (just the sort of news you want when you list your home for sale!!)

You want to avoid having your sump pump burning out attempting to pump out all of the ground water that might not enter into your crawl space from the pit.

No evidence of ground water or bedrock, but the plume is bigger than first thought.

I'm an environmental lawyer and seen lots of these. the costs vary depending if the tank leaked and contaminated soil or groundwater. a small tank with just some surficial or shallow soil contamination from overfills can costs $10K-$20K. if there is extensive soil contamination, the costs can be double, if groundwater is impacted, you could face much higher costs. i would not accept an offer until you have opportunity to take samples to see how extensive contamination. 

if the garage will be doing servicing, better to have sewer. chemicals discharged to septics have been a source of groundwater contamination resulting in cleanups.

So, yes liability is a concern.Your municiplaities concern is going to be ground water contamination.

Each has their difficulties and differences, but generally the easiest area to grow is the High Plains (deep sandy loam soil with calcareous base, hot days/cool nights, flat topography, low humidity, good groundwater and rainfall).