To answer your questions...
Replace all tires as they are 2007/8 production date, however thread is still very good.
Here in the US, the tire industry generally says 6 years regardless of wear. Might be shorter window where you are b/c of heat - You're call but ask around at another tire shop/mechanic.
Re-flush brake oil due to age.
Should be done every 2 yrs regardless of usage/etc. I'd go ahead - but this is a fairly easy DIY and lot's of detailed instructions here
Change brake pads, while the pads are still looking OK.
If pads measure ok, pass on this one
Replace engine belt because the car is almost 5 years old..
Pass on them doing it. This is one of the easiest DIYs and parts cost $25 in US and DIY takes 30min. Generally it's a visual inspection but for a small amount of $, why not do it while in there especially since it's 5yrs old. Keep old as spare just in case. This may also fall under heat deterioration as tires...
Next to this they noticed some oil sweat and told me that the sealing needs to be replaced urgently. Well I have never seen any oil on the road/parking or seen any dropped level. Should this really be done as this is a expensive repair? Engines are allowed to sweat a bit right?
Generally speaking IMNSHO, if it isn't dripping to the ground and level is constant, let it go unless it's coming from AOS but they didn't cite that.
Further they noticed some crystals under the engine what they suspect coming from a coolant leakage. They told me that the coolant pump need replacement. Why the pump not the sealing I asked, well it was better. Nevertheless the drippings are so tiny why should I bother?
You can investigate the water pump while doing belt replacement above but w/ only 30k kilometers on the car, it should be fine. Might have just been due to spillage or??? If no issues cooling, you should be fine but visual of pump when replacing serp belt will tell you more.
Good luck 