You're going to want to speak with a local engineer regarding the type of septic design, and then a contractor who installs these systems. Most of them are specialized and that is the bulk of what they do. As for grading, a good civil contractor will be able to give you some kind of an estimate if you've got a preliminary design. You also might want to consider trying to obtain a soils analysis for the property is one has been completed by the current owner. Might not be a bad idea to have an independent one done as well. There could be several other considerations that you may want to make prior to submitting an offer. A local Realtor could provide you good insight to some of the additional items you'd want to think about. Thanks and have a great night!