What he said about septic systems is very important. Just look for good red soil with sandy or loamy texture, not a rock pile, not tons of clay. You will also need about 36" of suitable material (maybe more depending on your state and slope). You ideally want a gentle slope to put it on, and realize the area needs to be cleared so cutting big trees will be part of the cost. A conventional septic costs 10-12K. If you need a pump it will cost $15000 pretreatment can go as high as $50,000 for a dripper system. Things you do not want to buy are land with poor topography (low areas, creeks, flood plains, or anything disturbed by digging) also gley soil is bad if it is very black or pale grey just stay away from it unless an evaluator tells you the soil will work. And realize your drainfield square footage will be influenced by soil texture and bedroom number. A slow soil like a heavy clay might need seven 100' lines for a 4 bed house. The same home in a nice sandy loam may need only 4 lines.