That's a very nice house for the money! Polycarbonate panels are the best material to use, in my opinion..
Heating is a very expensive option, especially in the middle of winter. I'd recommend just having a thermostatically operated heat lamp in there if you have to overwinter some
plants. It will keep it from freezing in there, but little else.
You can get a very early start in the
spring if you buy a propagating mat to start your
seeds on. These rubber mats come in a variety of sizes and have a built in t'stat and heating coils that provide a constant 75 degrees. They really speed up your
seed growth as well as rooting time for cuttings.
Wood is a very bad idea for the floor. First off..... the wood will stay wet most of the time, and rot before long. Wet wood also makes it hard to control the humidity. Wood also provides a safe harbor for insects and diseases. Using gravel and bricks allows you to hose the floor down and any spilled soil will disappear. A plain dirt floor will be slightly cooler, and again will hold extra humidity in the house.
You should have a small fan running in the
greenhouse to keep the air circulating. This keeps the
plants happy and prevents a lot of the condensation problem.
I usually added a cup of bleach to my cleaning bucket to control the mold, mildew, and moss.