What pressure should a hot and/or cold house water system be?

There is no set pressure for hot and cold systems. The normal gravity fed system which is the most common in this country would achieve a pressure of around 0.5bar in a two story house on the ground floor reduced to 0.3 upstairs. This system is mostly installed wrong in a lot of houses i.e. if a house has a bathroom, an en suite and down stairs WC a 3/4 cold and hot feed pipe will not cut it and fall short on flow rates and the end user will experience pressure drops across the system when working. This system is old in design (and some would say tried and tested) and is mostly if not completely obsolete every where else.

A more modern system would be a pressurized system with and un-vented hot water cylinder or combination boiler.
I would suggest if you do decide to go for this option is to use a break tank for storage because the local authorities have a tendency to reduce or shut down the water supply and if this happens you will still have a store of water to use, also if you choose to go with a combination boiler you should consider installing an electric shower as a back up in case the boiler fails so you can still shower.

The hot and cold supplies for the showers should be independent to everything else and this will eliminate the burnt bum scenario .