Water Softeners

Yes

Ion exchange types which run with salt, deal with scale and soften the water, so the salt cost is offset by detergent and shampoo saving.

Phosphate dosing devices like Combimate. These address scale but do not soften water.

Electronic, electric and magnetic devices do nothing worthwhile.

Very easy if you can do basic plumbing

Not really. It's a question of location and having a suitable route to a waste pipe or drain.

Yep!

I would recommend the ion exchange ones from experience with them.

The maintenance costs are pretty much just the salt, the salt consumption will relate to the amount of water you use and how hard the water is.

We had our last softener for about 25 years and only changed it because we wanted a meter one, and also needed a smaller unit when the kitchen was redone.

This does also depend on where you want to put it, as you will want to have hard water at the kitchen sink (for drinking) and also hard water to the outside tap.

You basically have the mains water entering the house, this then goes to the softener and also to any places you wish to have untreated water. Then, from the softener, you have the things connected you want to use soft water with (Water tank in loft, combi feed, taps you want mains pressure cold on (Like the Kitchen sink - we have three taps, one mixer with soft hot/cold, the cold being at mains pressure, and also another single tap for drinking water)

The softener also requires power (usually) a drain, and an overflow (NOT connected to the same pipe as the drain, or you will flood the place!)

Remember you need to fill it with salt, so a place to store some 20-25KG bags, and an easy route to take when filling, if the bags are not near it.

There are two types of Ion exchange (I think), one that generates based on a timer, so if you start using more water than it is set to, you will start getting hard water through, or if you have it set too high, you will be wasting salt.

The better types have a water meter, they measure the amount of water you have passed through it, then only generate when needed (I have just changed mine to a meter one, and it is using a lot less salt than the old one!)

If you do get one, make sure you fit a bypass pipe and also a non-return valve to the input (The instructions should tell you about this)

Sparks...