If you have it installed by an approved Viessmann installer, they increase the length of guarantee. The terms change from time to time but last time I looked, for the model I looked at, it increased from 5 years to 10. Have a browse of the maker's website. My own boiler is 16 years old and has only needed one (free) repair when they issued a revised design part on extended warranty. This may be why most local retailers don't stock Viessmann spare parts and they are sent out overnight. Your experience may vary.
I went for a 100 because it is simpler and has less to go wrong.
A heating engineer can calculate the heat loss of your house by taking the dimensions and the construction methods, especially insulation and especially the walls because (except in a bungalow) they are the largest heat-losing surface, and its location. This will tell you the power of boiler you need to heat it.
If your house is not very warm, it can be that your radiators are too small. if they are "too hot to hold" all over (top, bottom, sides and middle) and the room is too cold, they are too small. however, if the boiler is running at full power (you can hear it) 60 minutes in every hour, and the radiators are not fully hot, your boiler is too weak.
it was very common for builders 40 years ago, or even 20, to fit undersized radiators, partly because it reduces build cost, and partly because standards assumed people were hardy and had good supplies of woolly vests. Modern standards of comfort are higher.
A skilled heating engineer can get a good impression of radiator size the moment he walks into a room, but, again, you can calculate it.
You may also benefit from a larger, unvented cylinder, which re-heats fast and can supply unrivalled HW. If your existing cylinder is white, it already is one. It may need a bigger supply pipe, but, again, a heating engineer can measure your incoming water flow.
£2300 is quite cheap.