Didn't know you could buy furnaces at HD, except an installed package from their contractor.
There is a lot of labor, especially working in a crawl space or the attic.
If this were a primary residence, we would give you the usual lecture about doing a manual J heat load calculation, and having a very experienced HVAC contractor design the flows, the duct sizes, etc. Given your proposed usage, "winging" it will probably keep you from freezing on the occasional weekend.
Especially in cold climated, the ideal location for heat outlets is on the floor, because of the basic fact that heat rises. Here in S. California, most homes are done with heat outlets high on the wall, partly because it is cheaper ( ducts are in the attic, not crawl spaces or basements) and partly because high is better for the cooling season.
HD will give you a free estimate for the job. That would give you a starting point. This job has a lot of labor, and they will not break out the specific equipment costs, so it can be hard to compare.