Difficult call regarding the small bore pipework, swale. It was never very successful since it obviously takes only a small amount of sludge to bung things up. As the others have said, a good flush through, and it may well be Ok.
I know it's easy for me to say, but since the whole house is being decorated etc, then I would take this opportunity to run new 15mm pipework throughout.
At a guess, for a 4 bed house, I think I would go for at least a 35kw boiler. Oversize rather than undersize every time.