Just a little Question, If your not going to have any Fish in there why turn the lights off??
And also, if the garage is freezing cold, your going to find it hard to get ur DIY Co2 to pump much Co2 out, As the yeast lives and breeds best at bettween 20-26c.
Also if you've bought a good heater the heat shouldnt fluctuate much, especialy if there is a filter in there provideing circulation from the heater, Also i would reccommend adding a complete lid (if you have not already stated you would) as you will get a terrible amount of evaporation due to the water being alot warmer than the outside tempreture, and as you said you will not be out there everyday checking.
Personally i think a BN or two is a great idea, No worries about feeding, keeps the tank clean, and will keep algae from ravaging your plants, although you will have to gravel clean once every couple of months, you would have to do that anyway, because of the snail excresion. And it provides extra fertilisation and keeps the water conditions good by replenesion bacteria every water change etc etc
Shae