The bottom line is to buy the house you want and need. If it happens to test high on radon, have a mitigation system installed. The concern over this should not cause the average person any lose of sleep in a house sale.
I went the other way with my self build home. We do have high radon in the area, so high I could not ride my bike to work at a reactor site and then go to work in the control room area. Radon was getting all over my pants legs just riding along the highway. What I did, as a new home builder, was go in when the drain tile was first placed around the inside of my foundation footing. At that point a layer of gravel had been laid down under what would become my basement floor and a 4 inch perforated drain tile had been installed to carry away any water. Since the radon comes out of the ground I just penetrated that drain tile by cutting an X into the top of it and inserting some 2 inch vent pipes. I used PVC but any adequate vent should work. Then I routed the vents to a place above my roof line, as I might have done for a chimney install. The end result is a vent that has the wind blowing across the top to draw a partial vacuum and that connects to the space under the basement floor to intercept any radon before it gets inside. If the wind dies down to nothing, the vent allows gasses in the drain tile to move unrestricted to the ultimate downhill discharge point which is in a creek bed well below my foundation. With radon being heavier than air that means that when the wind stops and there is no wind induced draft the radon can drain away as if it were water. Either way the flow path to the environment is easier than a flow path through my basement floor.
retest again to confirm, better yet set a long term alpha trac for 91 days to a year, this will eliminate some of the natural swings and give you abetter yearly average
pay a pro to test, consumer based models are not very accurate in my experience of people using them. make sure they print an hourly reading so you can see the changes.
that would be considered a passive system and while they do have some effect, you should test to make sure. Here in Ohio we have several cities which require passive systems to be installed during new construction, most still test high, need to add a fan in the attic, connected to the piping to create a suction. might have to add a one way flapper where your tile daylites in the yard to create enough vacuum.
