Here is my story.....
Ok so we bought our house about 3 months ago we right away noticed dishes arn't getting as clean and the water tastes a little different and we figured we had bad water. One Saturday morning I woke up to a nice cold shower becaues the calcium build-up on the elements shorted them out. I drained the hot water heater replaced the elements and the sediment in the bottom of the hot water heater was terrible I cleaned it out the best I could. So I now I know I have really hard water as we had the sellers replace the hot water elements a week before we moved in and it only took 3 months.
To my questions.....
I have no experience or a clue on what grain system to buy. It is just 2 adults averaging no more than 3000 gallons per month or less. I would like to get a system to withstand 2 adults and 2 children at least so lets say 6000 - 7000 gallons per month. How do I gauge on what size system I need?
I'm am mechanically inclined and my father is a plumber so I am not worried about the installation part.
Is it safe to drink/cook from the softener if I pipe the entire house through the softener?
At this point with all the terrible build up in the hot water heater I plan on getting a new hot water heater at the same time as a softener and redu all the piping at once because my 50 gallon hot water heater is probably 40 gallons from the sediment build-up I'm sure.
They advertise that the water softener can save money because the water is not as "thick" and will not use as much energy to heat in the hot water heater is this true? Has anyone noticed a difference? If for some reason its not good or you are not suppose to drink water from the softener what if I drank the water directly from my fridge which also has a filter would this then be fine?

Sorry I am new to the water softener stuff. We are also thinking if we have to spring the money for a new hot water heater to go with a on-demand hot water heater or "tank-less" anyone see any real savings with tankless heaters?

Thank you for your time.