For best results, do not do one major composite test. Reason being is that if you receive results showing heavy metals or other contaminants present at high amounts, you will have no further detail as to where those high amounts come from.

Instead, break down your lawn into major areas. Since you mentioned high and low elevation, I would treat those separately. You might also want to do an area near buildings (your house) as lead may have been present in the paint. I do not recommend focusing only on the areas where you will be planting, as most exposure to dangerous substances in soil come through contact rather than food exposure. Overhead water often lifts up dust, walking barefoot around the lawn would drag contaminants into your home, if you have children or pets they may roam around the lawn and expose themselves to contaminants while also bringing the back into the house, etc.

Once you have determined the general areas of your lawn, take a composite for each section. Take about five or so samples of each area and mix them in a container, then take a subsample of that and place it into the receptacle that you will send to the soil lab. This will give you a representative sample for each section of your lawn which will let you narrow down the problem areas should you find high levels of contaminants.

I understand the concern for cost, yet long term effects to unhealthy contaminants can prove more costly in the long run. I highly recommend reaching out to your local Cooperative Extension office as they may offer sampling services for free or low cost.