While a gas powered RO works fine (mine was gas initially) if you can do electric, you will like it better. Far quieter and can be run indoors. Gas needs to be set out in the fresh air to run.
The RO must be protected from freezing or the membrane(s) can be ruined, along with other parts. A post above suggests 2 light bulbs, I started with 1, then 2 and finally used 3 at 100 watt on a low temp line volt t-stat. After doing that for 3 seasons, I put in a 8,000 BTU in wall propane heater with a t-stat, way better. Mine runs off a small bulk tank (50 gal) which runs several things in the sugarhouse (that RO room heater, a finisher 2x6 size, a weed burner torch as a fire lighter, a instant hot water heater and it used to run a propane fired canner before I got a water jacketed canner) Doing up to 180-190 gal of syrup a year I only use $120-130 a year in propane.
60A will run a small RO, but as you grow you would soon need to upgrade the elec. service.
As far as sap temp, in use you can RO any that is not frozen, the Deer run RO's are rated to process the stated flow rate at 37-38 degrees, if it's colder a little less, if warmer, a little more. I'm not sure where you got the 35 degree info.
On my Deer Run, a 250, I get pretty much 250 GPH at 38 F, about 230 GPH at 34F and about 270 at 40F That's about the common extreme spread. Those figures are for when I'm running at 270-275 PSI which removes about 70-75% of the water in 1 pass. If I have over 2% sap I start boiling as soon as I get about 30-35 gal concentrate ahead. If I need more concentrate I can turn the pressure down and I get faster concentrate flow, but at a lower sugar %. I also often run a second pass for higher concentration by sending concentrate to my head tank, then when I have enough in the head tank, I run recirculate to increase the sugar %. With a Deer Run the highest you can go is about 14-15%, on units that can do higher pressure can achieve 20%+.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.