You need to do some googling. There are plenty of web sites that can educate you better than I can. However, some comments from my experience:
I never felt that the treated water tasted salty.
I had 22 grains of hardness. The metal spray arms on my dishwasher were white from the calcium deposits and clogged regularly. After installing a water softener, no more issues.
A water softener should be sized based on the number of people, overall water usage and the water hardness. Pipe size mainly determines the controller valve inlet size and the max flow possible. If you plan on supplying soft water to your pool, you will need to add that to your overall water usage. You will need a larger unit if that is your plan. Most installations are set up to only treat inside water usage. If sodium is a concern, you might want to run an untreated supply to the kitchen.
I seriously doubt that hard water will ever clog your pipes. It will cause buildup where the water can evaporate such as in the dishwater, toilets, sinks, faucets, etc. I don't know about the water heater. I suspect you could get some buildup (not a foot!) and lose some efficiency and maybe a little bit of life.
They will tell you about all the money you will save on cleaning products. In reality, you should be able to use less detergent in the dishwasher and washing machine but the saving are not that great. Your shower doors will be easier to clean.
My first water softener was from Sears for $500. Last about 7+ years until the resin tank started to fail. Worked fine, never clogged. I would not recommend Sears. The second unit was purchased on the web. This one had a Fleck valve with demand driven regeneration. Cost about $800 about 4 years ago and should last 10+ years. Very easy to install but you do need power. The support from the dealer was excellent.
You will need way more than 15 pounds of salt.
Regeneration is usually done at night. During that time (roughly 1-2 hours), you will have untreated water. Who cares if at 2 am that the water is hard.
These systems tend to reliable. If the controller fails, they can be repaired or replaced. The resin does have a life span and can also be replaced.
My recommendation is to consider buying on the web or maybe a local dealer. Find a dealer that will help you properly size the unit. You do not want to vastly oversize or undersize the unit. From what I remember, the unit should be sized so that regeneration happens every
7-14 days. Stay with a high quality controller (Fleck is one popular brand). Install it yourself or hire a plumber or handyman to do for you. Make sure that you have a means to bypass the system if service is needed. A lot of controllers have this as part of the system. Unless there is more to the story, a $6000 dollar system is overkill.