Cooling the tank down won't be a problem as it won't get warm in here at all ( holland is way cooler than france, the natural habitat of T. Marmoratus), at least nothing T. Marmoratus won't be used to. I'll probaly get them the 10th of september anyway. Summer is over here by that time.
To prevent escaping I indeed have to take some measures, I'll probably cut and paste some glass to the sides where they could escape, but the tank itself is pretty escape proof, Maybe I'll post some pics how the tank currently looks without the cover on top of it. There is already some glass pasted at the risk-full places, but just to makes sure i'll make it 100% escape proof, I'm familiar with their alcatraz-like escapes. The hole is made to feed the fish that were in the tank before my turtles made their entrance. I don't have to cover that because unless a newt can walk upside down, they can't escape through that. The crack in the cover is because the wood is old and bent a bit, I'll use screws to fasten it again.
I'm still waiting for some opinions on the soil, I won't install it before someone replied on it, Because I don't know if the plant-feeding used in he soild can be harmfull for the newts.