On the other hand my neighbor is still perking the soil and groundwater 3 years later.

That's just one example of why you must identify the source of the water.If your problem is high ground water (rare in the West ), then really truly the correct solution is an EXTERIOR French Drain.

Should I pay to have the soil & groundwater tested or am I going down a rabbit hole?!

This is the only way to know the extent of the contamination and whether groundwater is impacted, requires long term monitoring, etc.The common approach is this: perform a Phase 1 for $1,500 as one of your final Due Diligence activities and if it does not pass then walk away.

Houston is technically BELOW sea level and has a lot of ground water.

French drains (inside drains in the floor) can help in certain cases with groundwater, but often don't do well with hydrostatic pressure along the foundation walls.  

Ground water allocations were virtually non-existant for the past 4 years.  

Alex, you may already know this but when buying a MHP that is not on a City or County permitted Public Water System and has its own drinking water source such as a groundwater well(s) then EPA law (that most States Env.

We had to shutdown a property once and became listed as a supersite clean-up as a result of a dry cleaner chemical leaching into the ground water. 10 years and still waiting for Federal clean up dollars.

You'll want to have sub-slab sampling of the soil gas to see if there is a potential for vapor intrusion as well as groundwater.