The offer is valid and it works on 3G coverage only (also HSDPA in many places), I'm writing you right now with my laptop and the Naviga3 option using an old Motorola A835 as modem... and it works quite well. I suggest you to put your phone on 3G-only, so that it won't use TIM GPRS network...
Buying a 3 prepaid card should be not a problem, exactly like with the other operators. There's also an offer right now: you pay 20€ and get 20€ of credit at once, plus two bonuses of 5€ in the next two months (offer valid till 30/4/07).
To activate the Naviga3 option you have to call the toll free number 4321, but I don't know if you can choose English as language with 3... anyway, in case let me know that I can help you with Italian.
Validity period with 3 ITA is a total mistery!
I've 3 prepaid cards and they never expired, even if I never refilled them! Now with the new law it appears that they should officially never expire, but with 3 you never know...