I have a rheem two stage unit myself and originally had a single stage
thermostat. I went for a nice honeywell two stage thermostat to take
full advantage of the unit.

A two stage thermostat controls what heating is necessary in the house
to get it to the preprogrammed temp (either high or low) The
problems without a two stage thermostat is that the thermostat can
short cycle a furnace. If it takes 12 minutes to heat the home and it
runs on low for 10 and then kicks to high for 2, thats not good. No
point in that one. With a two stage unit, generally if there is a 3
degree difference is what the house temp is and what you want it to be
it should kick into high first, bring the house to almost temp then go
to low and bring house to the desired temp. From there if the temp
need to be maintained it uses low to do it. I can also set how many
times I want the unit to allow the cycling of the furnace on and off.
I have to have something go on 6 times an hour. (I feel there is more
wear and tear) A good thermostat can adjust this setting.

As for not wanting to connect it because of the existing wiring??? Bah!
Most wires that go down to the furnace are 5 wires. That's all you
really need for a 2 stage unit (24volt (that's 2) furnace controls
(another 3)) If you dont have it, you can eliminate the 24volt line
and go with a battery operated thermostat.