Didn't realise that was the only boiler on the market? Budget might stretch that far? Do they need that size boiler?
This is where a lot of customers get ripped as a plumber will always recommend a boiler they always use that's not always the cheapest.
Boiler should be around about the £700 mark, and installation if straight swap should be between £400-£600.
You'll probably get quotes around £1400.
Get a few quotes first to compare and play them off each other to try and reduce costs, and see what boiler they are quoting for, then go to a...