friend, i drive a 1983 300 sd w126 day in and day out
365 days a year.... and during the summer it runs hot wit the ac running all the time so i take out the thermostat.... and during the winter
i put the thermostat back in and i put card board over the radiator and it never gets warm... never...

personnally i just put svo right into my fuel tank and in the winter
i dilute it with diesel... and i never have a problem... never....

but if your going to have a seperate tank the best way to do it is to warm it with a block heater and an electric battery warmer.... just plug it in... and when you drive you will use the hot water from the engine...but instead of that i would use a 12v warmer.... and just forget about the water... the most important thing is the relay switches that will switch from diesel to svo.. i am sure if your going to this trouble your also going to build a switching system that will allow you to start off on diesel.... these things are notorious... and often pump diesel into the svo or svo into the diesel...