I’ve been reading the questions of similar situations on this site for days but each scenario differs from mine in a way that leaves me scratching my head wondering which bits and pieces of info I should apply. I figured it may just be worth explaining where I’m at and hoping that some understanding guru out there will point me in the right direction. 
I recently bought a piece of property in Maine that has a 16×30′ concrete slab on it. We would like to build a small cabin on the slab but we’re not sure where to start. First off, I’ve never really built anything so bear with me. Second, keep in mind that it’s intended to be a cabin (built on a budget), so not everything needs to be textbook best practices or anything. The slab is not insulated. Can we use rigid foam and plywood directly on the slab and cut our plumbing into the concrete? And if so, how would we deal with a load bearing post? Can non-load bearing walls be built on top of the floor? I’m wondering if some thermal bridging wouldn’t be a big issue since the cabin will be small and the woodstove will probably roast us out regardless. The other option I see is framing out a floor on top of the slab and using this space for the plumbing.  It seems like rigid foam and plywood would be the quick and dirty way of doing it, but I figured I’d just put it out there and see what advice may come in. I appreciate your time.