Just keep in mind most portable space heaters are 1500 watts. Which if
you do the 10W per sq foot rule should be plenty for your
requirements.

My mistake. At 500 sq ft, you need at least 5000W of heat. A 1500W
space heater is not going to cut it.

You need to look at something with fossil fuel ( gas,oil, propane), or
a 220V heat source which depending on your utility rates could be
expensive.