Hey there poli fan-
I'll try to help where i can.
a. You can't get tickets to the Curva unless you have the CR card, and it's not worth it just for a single match, especially when it's an inter game. Get seats in primo\trezo blu if you wanna sit next to them so bad.
b. Get tickets ONLY from the inter website. I'm going to the match myself and after talking to one a the tickets sale representatives in Italy she told me that if i want to be on the safe side- i should get tickets only from their site. General sale opens in a week (All the info is on the site).
c. Theres no best airport, lol... usually the cheapest flights are from capital to capital, although some charter companies can offer you lower prices because they frequent a specific airport. Check Kayak\ Skyscanner\ Expedia to compare prices. If the lowest prices are landing in Rome (which i would guess they are), you can easily get to Milano with the italian train- it's fairly cheap, and fast. look up 'trenitalia' on google- thats the national train system. Rome-Milano should cost you about 50 Euro with a 3-4 hour trip duration.
d. For hotels and stuff- go on Booking.com to check general prices. Keep in mind that there are several apps that gives you 50% off if you book 24 hours in advance (without knowing the name of the hotel). Also, Booking offer hugh price drops on random hotels each day, so i would suggest booking only a day before you flight\ a day before your'e visiting.
If you need anything else just ask.