75K btu might be a bit big actually. Ideally you want a fuel burning heater to be running 100% of the time on the coldest day of the year. Why? Because fuel burning heaters that run for only short periods never achieve peak combustion efficiency, plus with the fan off for long periods the hot air rises and you're cold, so you're unhappy or you turn up the thermostat and use more fuel.
Ceiling fans can mitigate heat stratification, but it's an extra expense to buy and run. Better is to size a heater to have it running more often, it's actually cheaper and more comfortable.
But, a 2 stage heater is ideal! Low fire means the fan and burner are running longer which means the heater is spending more of it's time at steady state efficiency, plus the fan is on longer reducing heat stratification. Oversizing a 2 stage heater is less of a concern, as it can spend most of it's time on low fire, only bumping up when you want rapid heat gain after opening the garage door or bumping up the temp prior to working there.
So, for only $200 more you should ABSOLUTELY get the 2 stage heater, and 75K btu will be great as it'll allow more rapid heat rise when you need with none of neg consequences of oversizing a single fire heater.
