Odd I seem to have seen something like this before.
Silent generators are normally expensive. The WisperGen with it's wobble shaft
is very quiet and they have been installed with central heating boilers to use the waste heat being an external combustion engine it can run on multi-fuels but where is seems to have a problem is with the price.
There are two methods used with internal combustion engines to reduce noise but non are really silent. One is sound insulation the other is a low engine speed.
Most alternators need to rotate at 3000 rpm simple 50Hz times 60 seconds = 3000, there are models running with 4 pole at 1500 or 6 pole at 1000 rpm but as you slow down the engine so the engine has to be larger.
As to heat again depends on engine type. Water cooled lend themselves to heating as well as producing power. Air cooled one has a problem ensuring no fumes also enter the building.
Liquid fuel heaters will remove the need for a lot of power and once you are only worried about lighting the it's a completely different ball game. Using LED lighting very little power is required.
Most people want them portable so want them small and the more you make the cheaper you can make it so as you start looking at lower revving units the price goes up. Main reason to reduce speed if to reduce running costs and some diesel driven units have a tick-over setting so at no load they tick-over and only rev up when something is switched on.
But the popular caravan site answer is the inverter generator. What the inverter does is remove the need for the generator engine to run at a set speed so speed increases as the load increases. Often they also have an option of running two together.
