You need to get someone who understands heating design - this does not mean you lookup "plumber" in the directory. Some plumbers can also design systems, many cannot.
You have two things to consider :
1) The heating load. There are well established ways to calculate that - some guess based on size and assumptions about losses, others are more scientific and use calculations based on size and thermal insulation (U) values - your architect will be able to do that for you assuming you employ a competent one (he'll be doing most of the calcs anyway to satisfy building regs I believe).
2) The DHW load.
Here you don't work out a kW load, you need to guesstimate the water requirement and then you can apply trade offs between instantaneous power available and storage requirements. Eg, if you have a lot of heating capacity available, and the right sort of tank, then you can use smaller tanks. Alternatively, you can have larger tanks (to satisfy demand) and reheat them with less power over a longer period. Don't forget that some people will be used to long power showers - so across all the rooms you could have a lot of hot water demand (I've stayed in hotels where they've found out the hard way that their system is inadequate).
Personally, on the basis that boilers do break down, I'd install a pair so you have some redundancy. Also, consider a buffer tank and modulating pump (or pumps) - this makes a big difference to noise as the buffer will decouple the flow rate requirement (high) through the boiler from the flow rate requirement (may be low in warmer weather) through the rads.
But a good heating designer will be able to advice on all this - and explain the different options and the pros/cons of each. There are many options - and things like measuring your dynamic cold water flow rate and checking your gas supply capacity will also feed into the decisions - eg it's no use designing for a high power rapid reheat DHW system if the gas supply isn't up to the peak demands.
