I'm in favour of getting one from your local specialist. Phone and say "do you repair water softeners?"
If no, try another.
Then ask if they deliver salt in 10kg bags, or in blocks.
If they do all that, ask if you can call and look at the softeners they sell.
The machine is very simple, made up of standard parts, but you need someone local. IME they will go ten or twenty years. Last time I bought a recon exchange because it was better value than a repair.
A big one has a larger container so needs filling with salt less often. Consider how you will carry the bags and tip them in. Mine is in the garage and easy to get at. Some places only sell 20kg or 25kg bags.
They are heavy when full so stand on the floor, not the bottom of a cabinet.
Do not carry salt in your own car because even a single spilled crystal will eat a rust hole through the body. Look in the delivery van and you will be shocked. Also do not put the softener where you can't sweep up spillages and sponge clean easily, or where salt may contact tools, bikes etc.