Ciao Jacopo
Welcome among us

I wished I had the money to afford a Morgan at your age. It took me more than 30 years, since I first saw one (at the age of 15), until the day I was able to call one my own. (at the age of 48)
But that's OK... Let me try to answer your questions
- is the soft-top reasonably water proof...
Yes, but you will be surprised, where the water sometimes enters the cab.
...or should I use a hard-top when it's raining?
Don't know, but I can imagine that as very unpractical.
- going on higways at 70 mph is the car noisy?...
Yes.
... I mean, will I have to take an aspririn afterwards?
It depends...
- having the Roadster the discs only on front wheels it is suggested to install them also on the back?
The brakes work pretty well
- is there any specific option that you suggest to install having the idea to use the car nearly everyday?
In Italy... maybe the air conditioner. The lower part of the cabin gets pretty warm, because it's right next to the engine, and with high outside temperature there is no way to cool your feet.
- is the external luggage rack suggested or the back of the seats can carry some bags
There is very little space behind the seats.
Good luck, and please, keep us informed

Salut,
Ivo