25 vs 30kwh combi boiler - running cost difference over 10 years?

Plumbers have a tendency to oversize boilers. However, as it's a combi boiler 30kW is not that big. Although the rated power is 20% bigger, the additional running costs should not be that much greater. The boiler will start at full tilt when you start to draw water, but it will modulate down to adapt to the flowrate. It's sometimes a problem with combis that they don't deliver enough heat. With the 30kW there's a better chance of you not turning a shower cold by running a hot tap at the same time. I think the length of your shower shouldn't matter. There is no HW tank. The boiler should be able to deliver water at a steady state for however long you want it.

Sorry, I don't know enough about individual models to say which is better. But check with the plumber that the boiler he's proposing has the full manufacturer's warranty (typically 10 years). 

The price will depend on the amount of work involved. The website below gives some guide prices. A direct like-for-like combi replacement with the Greenstar 30si is "from €2590". Ideally you would get alternative quotes, but it's so difficult to get even one registered gas installer at the moment that, unless they're outrageously expensive, you'll probably have to go with the one you've got. When we got our boiler replaced a couple of years ago, to the best of my memory, the installer applied for the scrappage and the price he quoted was net of the scrappage allowance.