Ask if the owner had replaced the units under the recall notice:
http://www.kidsource.com/cpsc/InWallHeaters.recall.html
If so, the units might be safe by now.
Check the cost per btuh delivered between propane and electricity as electricity is usually less expensive in your area. If you install ducted systems include a very effective pleated filter in the return. The Air Bear media has to be changed twice per year, but it prevents dust and allergens from entering the ducts to create allergy problems. $100 for high-efficiency filters is a reasonable investment in health.