you might get through a 10-20 kg of salt a month.
I get the 10kg bags delivered, ten at a time, and store them in the garage or carport. Last time it cost me £30. You can also get 20kg or 25kg bags which work out cheaper, but are hard to carry around and tip into the bin of the softener. Some softeners take block salt which is considerably more expensive, but easier to handle. I repeat, never carry salt in your own car.
I have had a few softeners in different homes, and they typically seem to last about 10 years between repairs. Last time I got a reconditioned one in exchange from a firm in Essex which I thought was good value.
A softener is very heavy when full, so needs to stand on the floor, not the base of a cupboard. It must be close to the incoming mains water supply and a drain. As you don't need to access it often, you might be able to put it in the corner of the kitchen, and roll out an appliance or lift a hatch in the worktop to fill it with salt. Mine is next to the stopcock, in the corner of the garage. It is important that salt or salty water cannot spill or splash onto metal tools, or drip onto a concrete floor.
The construction of a water softener is quite simple, and the components are widely available, so some local firms assemble and sell "own brands" if they have a workshop for repairing or servicing softeners, assembly is quite easy. They all seem to use "Fleck" brand mechanisms. It seems to be the industry standard, and contains all the working parts.