I've a block heater in my 240D that's installed in one of the freeze plugs. Had one in my old 220D that I bought from JC Whitney and installed it in the middle of a radiator hose. Both worked fine and needed only to be turned on about thirty minutes to an hour before you want to start the engine. None of these had a thermostat. Your radiator and cooling system thermostat will work pretty good at controlling things there, but you'd be wasting electricity and element life to go this far.
As far as antifreeze. Almost any antifreeze, 50/50 with water (from tap) will do. Only if I was on town water with a bunch of chlorine in it would I buy distilled water. My favorite antifreeze is what you can buy from Mercedes dealers. It's the only colorless antifreeze, which is what ethylene glycol alone is, I've ever seen. It'll cost more, but I like that nice clear look to it.
If you want stuff more environmental, buy the Sierra stuff that's propylene glycol. Think they both need an expensive distillation unit to recycle, but propylene glycol isn't poisonous to humans and animals like ethylene glycol is.
There are some long life ethylene glycols. This is what my VW has in it. Won't know whether they make a long time propylene glycol.
Don't know what Fleetgard is.
Ralph