PJ is the source of all knowledge. I'd go with what he said.
We had an oil fire combi (potterton but I don't know what metal) and a water softener. The plumber wouldn't install a combi without a softener. What he neglected to tell us was that the new softener would de-fur all the pipework so all those pin holes in the pipes which were closed with scale opened up. Lots of leaks....
Softened water must not enter the heating system or main heat exchanger - however it can go through the plate heat exchanger which heats the hot water on a combi. This is on Worcester boilers - each manufacturer is different so check first.
Regarding the whole house lime scale protection there is one called Aqubion - don't know how good they are though. But they are fitted at the stop cock.
The Aqubion s15 is about £400 plus fitting. Which is cheaper than a water softener.
If water softener is the way you want to go you would need a high flow one on a combi with 22mm fittings so the flow is not restricted.
We have a Monarch high flow non-electric which we are very pleased with. It regenerates based on the amount of water used - so normally once a day. Blocks of salt usually last 3-4 weeks and cost £4 per pack.