You will need a full water analysis of the water from the new well. I always prefer to plumb systems in with copper. If you go with a fleck valve for softener they make a one piece by-pass valve that bolts right to the valve. You can sweat to this or use MIP to compression fitting that makes install very easy. To close down system at end of season just take valve off and siphon water out of softener tank through outlet manifold. We do this a lot in our area with seasonal cabins. Depending on the water chemistry you can probably go with a 30k softener system. Fleck 5600 metered with mechanical meter is a good basic system for this application. You can use a small
square (11x11x38) Brine Tank. This combo will take up about 2 sq ft. Same footprint as cabinet model with a system that will last you a very long time (maybe till the kids inherit the cabin). Use a 20" or BB pre-filter to keep up the flow rates.
I hope this helps. Let us know about the water test.