I first got a Permutit, which used to be thought the best, and when it eventually needed replacement, I got a reconditioned one. Permutit went bust and the brand name was bought by some other company who stick the name on a cheaper machine.
If you buy an unbranded machine, or a cheap one at B&Q, it will probably have to be thrown away when it goes wrong.
Culligan seem to be OK, but what I would suggest is that you locate your local water-softener company, who sells and delivers salt and repairs softeners (ring up and say "do you repair water softeners?") and you will find they sell, and probably also assemble, a preferred brand, which they guarantee and repair (the main working part is the meter, timer and valve, all in one piece, and I am told that almost all softeners use the same make; nothing else seems to wear out and the softener resin is bought in from some big chemical company). If you buy one from your local firm, it is worth paying a little more if you can trust them to repair it when needed.
Mine needed repair, on average, I would say at about ten-year intervals, when the "O" rings jammed with wear and needed to be stripped out, cleaned and lubricated, and new bits fitted. It is less than an hours work for an experienced technician, so I think very reasonable. The local firm also delivers salt sacks. I am not very weedy but I find the 20kg sacks quite heavy to lug about and lift up to pour into the machine. 10kg bags work out dearer.
Never carry sacks of salt in your own car, even a single grain falling out in the boot will eat a hole through any steel it touches. I have looked in the delivery van and at less than two years old, the floor is very rusty.
If you have a combi and no water-tank in the loft, buy the larger-bore connection hoses and valves, as they let water through faster. It will cost a little more
Fitting a softener is almost exactly the same amount of work as fitting a washing-machine for the first time.
Because of the heavy sacks you tip into it, put it somewhere easy to access. Many kitchens have an unused space in the corner of the kitchen where you can't put an appliance or open a door. If you are fond of kitchen-fitting you can make a hatch in the worktop that lifts out for salt-tipping.
The softener will be extremely heavy when full of salt and water, so stand it on the kitchen floor, not the bottom of a cabinet.