if the center court ticket is difficult to buy on the day of playing or not (just for early rounds such as 1st & 2nd rounds)?
Depends on where you would like to be seated. Court Side tickets are officially sold out throughout the week.
Under "GET TICKETS!" on right you can can see availability of the tickets and the best seats (PS/CS) are sold out. I cannot foresee the Nadal effect on earlier rounds but guess you will be able to get a ticket.
"A Seats-Non reserved" (4-7 Oct) is only for Center Court and for other courts you need to buy a ground ticket.
If you take りんかい線, and get off at「国際展示場」station, it will only take you 5min walk. Hope my understanding is correct, or 都バス and 新交通ゆりかもめ will be easier to get there?
Yes, I recommend 国際展示場(Kokusai Tenjijou) of りんかい線(Rinkai Line) or 有明(Ariake) of 新交通ゆりかもめ("Shinkoutsuu Yurikamome" or just ゆりかもめ"Yurikamome"). Then the Venue (Ariake Coliseum/Colosseum) is just over a crossover bridge over highway.
都バス(bus) option would be complicated if you don't speak Japanese.
(My alphabet writing for Japanese words above could be a bit different from what you see there.)
If you need further information, you can PM me.
Hope you will have a nice stay in Japan!