I want a thermostat that I can place outside, so that the heating unit won't turn on unless the outside temperature falls below 48%. What do you recommend? Is there anything on the market that's water/cold resistant?

Yes, just get a cooler thermostat and if you are controlling heat strips or gas heat break the "W" wire. If it's a heat pump you will break "Y" outside, but you will need a switch to bypass it during summer. These t-stats have a normally open and normally closed terminal. So, from common to one terminal will open on a rise in temperature and the other will close on a rise, and vice versa. Place it under an eave so it is protected from the weather and the sun. You can attach Seal-tight conduit to a junction box and the t-stat to make it water resistant. DrTom4444