I got my last one, reconditioned, from a person in Ingatestone, Essex, who advertises on ebay, and have been very pleased with it. I recommended him to someone else who lives in the area, who is also pleased. He does not seem to be advertising at the moment. They are usually in the region of £200-£300, IIRC
I would go for a metered one, that regenerates according to the amount of water that has passed through it, and it is worth paying extra for the large bore hoses and valves if you have a combi or megaflo.
The parts they are made from are pretty simple, and they mostly use a valve assembly made by Fleck, there are no other moving parts.
See if there is a water softener company in your town that services them, if so they will probably assemble or recondition softeners and offer a guarantee on them, and repair if need be. The cheap softeners from the sheds, you might have to throw away if they go wrong.
The repair prices you have been quoted look rather high to me.