All the "what's nice" talk is really besides the point. The OP is building new and wants to maximize the appraisal in a crazy market. The ONLY winning strategy is to save whatever money they were going to spend on these improvements and hold that cash until they close on their permanent mortgage. The cash will be MUCH more valuable than a barn or pond at that point. It is SOOOOOOO much easier to close a deal by plunking down $50,000 in cash vs trying to argue about the worth of a barn. Once the appraisal is over and the mortgage process is complete, then the OP can start with any additions. At that point, if you are cash flowing any improvements, you should just order them in terms of what you value the most.

Ground source heat pumps (AKA geothermal which is incorrect IMHO) used to be a thing. They are probably not such a big thing anymore because of low gas prices. You, however, don't have natural gas due to a rural location. That is a problem that I am very familiar with since I come from that location. If you are going to be in the home for some time, it may pay to go that route but the front end investment is high. You can heat and cool the space adequately with anything that you like as long as you match the output capacity to the building. I hope that you plan on a thermal break and insulation between the existing slab and the heat and what ever else you put above it. Ground source is an excellent way to drive in-floor radiant heat. Low temp water is what is called for and that is what ground source excels at. Heat pumps can also provide chilled water to power small chillers in each room if you want to avoid ducts. That kind of thing is done commercially/industrially all the time and can be done in homes. Why do you want to avoid ducts? As long as you keep them indoors, and have balanced flow, they are not a problem. Have you considered hiring a residential energy rater to help you out? They can do the calculations that help you determine if ground source is good and all the other decisions. Several models can be built and choices made. I really don't have a good head for your insulation numbers. If you construct tightly enough and insulate enough, it really does not matter how efficient or inefficient your heat or cool sources are since they will be little-used. You need some numbers to make those decisions. Local energy raters know what energy efficiency subsidies might still exist. Interview a hand-full and see what they can do for you. resnet.org bpi.org

It seems like some of your plans for cost savings involve family/friends being willing to do the work for you. That's great, but it seems like it would also apply if you build a more traditional style house; you don't have to have a pole barn to get help from an electrician in the family. You mention that things are more relaxed and you don't necessarily have to follow code, but remember that the code is there not just to bother and limit you, but to keep you safe. Blithely ignoring it can be dangerous. As Sophie points out, being on a well makes it even more important you make sure waste is disposed of correctly. Give your family the gift of modern, sanitary indoor plumbing, even if it costs more. Listen to Janni's point about a 50x50 v. 40x60 house. Rectangles generally lend themselves to better layouts than squares do. Then, price out your pole barn concept (built to code) v. a traditional stick built concept (built to code) in the same basic rectangle. In both cases, factor in the savings from of electrical work done by a family member, butcher block from the high school shop class, etc.. If the pole barn is still cheaper, go for it. You might be surprised, however, to find that it's not.

Wood stuck in the dirt won’t meet the building codes. Turn that into a pier and beam foundation or slab that actually will pass inspection for a home, and you add 30K-60K. 2x4 on 24” center with no tiedown straps or other bracing, won’t meet code requirements. Make that into 2x6, with proper bracing and strapping, and add 10-15K. $49 box store windows that have no double glazing and meet no standards any where turns into a 20K-40K upgrade to get even the minimum required glazing specs. Spray on adhesive cellulose at a R-5 won’t meet any insulation requirements anywhere in the US. Add 15-40K. Chicken sheds don’t require but a couple of hanging protected fixtures. An actual house requires a complete modern electrical plan. And lots of lights. Add 15-50K. Chicken sheds don’t have plumbing, unless it’s a rudimentary cold water supply only. Add another 45-75K, as this includes waste removal also. A “pole barn” is nothing but an expensive shed and an even more expensive house by the time you add all the things that a house absolutely requires. People who live in areas where they decide that they can skip doing health and safety code requirements are just paying with different costs than building costs. They choose to pay part of the costs with the health, safety, and comfort, of their family instead of money. Imagine growing up in a building that’s freezing and damp in the winter, with bits of breezes coming in through the zero air sealed and properly insulated windows and building. That’s your kids paying for a non code compliant building.