Sorry- this is long, but wish I'd known this info from the start!
When we renovated a derelict property we installed one. We didn't live there previously, so cannot comment on loss of water pressure, but we have a pressure booster anyways. Compared to my old home without one- its definitely noticeable. No streaks on bathrooms mirrors or screens, no build up in toilets, I use less shampoo, less washing detergent and dishwasher stuff- the list goes on.
You can generally find brands which take loose salt- which are actually small, compressed salt pieces about 50p sized or large compressed blocks like bricks. 1 company (I think Harveys?) had their salt blocks in a specific shape that only fitted that machine! Personally, I found the general loose salt cheaper and easier to find. MIL has the block type and her salt price has doubled in 3yrs!
We did get a fresh water feed with a filter in the kitchen and also en-suite upstairs (to save coming downstairs for a drink in the middle of the night). You can drink salted water, but I hate the taste. Its also not recommended for babies, during pregnancies and those with kidney issues needing a low sodium diet.
There are roughly 2 types of softner. 1 is set to re-set itself daily which uses electricity and water to do so- regardless if you have used any water at all! Something about ions and 'recharging' the system. The other type only does this recharge thing when enough water has passed through the system to need it. I 'think' the 2nd type can be slightly more money upfront, but less spent overall in the long run. It also depends on your household size. If you have a larger family, everyone showers/baths daily and you do lots of washing and using the dishwasher, then the 1st type might be fine. We bought kinetico in the end, after lots of demos, reading reviews etc. Cheaper options include monarch from screwfix, but I don't know what type they are nor quality etc.