I'm 3/4 through this process but it is too hot up there now.
I have standard 100m (4") fibreglass insulation between the joists. I've topped up with Wickes 170mm insulation. For my boards I've used these:
www.toolstation.com/shop/Access%20&%20Storage/Loft%20Ladders/Loft%20Flooring%20Legs/d110/sd3303/p44016
I've used 18mm tounge and groove chipboard flooring but its heavy and a size to manouvre up through the hatch and then around all the truss braces. You can use specific loft boarding which comes in smaller pieces.
I've had to manufacture a deeper loft hatch to take the extra height as the boarding and insulation will come right up to it.
As for extra weight. Roofs are generally built to withstand weight and I have sufficient internal structure to support all the ceiling.
Damp shouldn't be an issue providing the loft space is ventilated and you are not letting warm damp air into the loft space. The lofty hatch should also be well insulated. I'm going to use Kingspan for that.
It is quite a time consuming ongoing project due to the size and amount of carp up there. + some of my ceiling joists are 100mm and some 125mm so I have had to screw pieces of 100mm to the 125 mm to keep everything level.
When its done I could get a massive model railway up there - but maybe not :)
You may also need to consider pipes tanks and extra frost protection leaving space under any tank you may have for warm to rise.
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