Your instincts were correct. Do not run the compressor with lp switch not made. What you did indicates, but does not confirm that you have a low charge. You need to connect a gauge set to confirm. Since the system has never been touched, it is possible that you simply lost enough refrigerant through microscopic leaks that you could recharge and be good to go. There are some steps to take first.
Look everywhere you can get to for an oily residue at the compressor clutch and all connections. That's your preliminary search for leaks. Then connect a vacuum pump, pump down for ten minutes or so and close manifold valves and wait 30 minutes. If the vacuum did not go down then you don't have a large leak.
If you don't have a large leak, you should be able to add 1 0r 2 oz. of mineral oil, replace the receiver/dryer and recharge. It would not be a bad idea to put UV dye in the system at the time so that you can easily find a leak if you lose the refrigerant.