Think will try and keep it downstairs as just seems easier and closer to the gas outlet.Fine as long as you have good access for servicing.
Regarding pressure etc, you'll soon find out if it's working or not due to pressure loss. They also have safety valves so nothing to worry about. The boiler will stop working on the heating side of it doesn't have pressure, so just pop up in the loft then. Nothing is fool proof but the reality is boiler don't leak that often, and in theory a once a year service should be enough and then a quick check every time you enter the loft.
The loft will still require pipework to be insulated. No need to worry about heat in the loft either won't have an impact on the boiler.
If your having large scale works done look to oversize the radiators and install an opentherm thermostat, this will enable the system to work at a much lower temperature and use less gas.
Make sure the boiler isn't over sized for the property when switching to combi boiler installers just tend to throw over sized units in.
I wouldn't personally go with ideal, but at the end of the day probably best to stick with what your installer suggests as they will likely be servicing it and know the intricacies of that brand and model, and sometimes they are affiliated installers which helps the warranty process aswell should you need it.
Never heard of Ideal before this, always had Valiant.
They quotes 32kw model, it's 3 bedroom detached house with one bathroom that will have shower and bath in it. There are currently 9 medium sized radiators but will be getting extension done so 32kw sounds about right.