I assume that you are talking about 4x8 raised beds or something similar. In that case the popular option would be to use pressure treated 4x4s. There was some concern in the past as to the safety of using pressure treated lumber around vegetables and children being that it was treated with an arsenic compound. To my knowledge, due to this concern, most manufacturers switched to a less toxic ammonia based chemical. Thats for the insecticide part. The main fungicidal agent, copper, reacts with steel so galvanized fasteners (the smooth, shiny ones, not the lower grade duller, crustier looking ones) or stainless fasteners are recommended.

Being that this would be for a church I assume you are working with a limited budget. What you could do is use 2x6s, which are a couple of bucks cheaper than 4x4s, sunken a few inches to negate the use of fasteners. Then just dig the grass in to make the most use of the topsoil that you already have. I would think you should be able to come across some form of free manure being in South Dakota. It should be well rotted or dug in fresh at least a few months before hand or else it will "burn" plants, especially seedlings.

Of course, a less thrifty and less labor intensive option would be to use 4x4s, toe nailed together and then thrown on the ground, the center being filled with bags of garden or potting soil from the store. Topsoil, in bags from garden stores, although nutritionally not the best, would probably be alright too, depending on what types of plants you want to grow. What I would avoid is topsoil sold by the truckload by landscapers and the like, which is usually just a "fill" and contains alot of clay and rocks.