JR Pass

I will be traveling in Japan via 7 days JR pass. Can I have an advice regarding the JR pass:-
1) Can i purchase the pass at Kansai Airport when I reach airport. Can I pay by credit card.
2) From Website, I understand the pass is valid for any JR including Shinkansen, Limited Express, Express rapid & local. But if the desination that I intend to go require me to take Limited express train, does I need to pass extra even I already purchase the 7 days Pass
3)Any suggestion for Onsen between Maibara to Nagoya station.
Thank you.

I assume you refer to the nationwide Japan Rail Pass as opposed to the various regional JR passes that are available.

Can i purchase the pass at Kansai Airport when I reach airport.

No. The pass has to be purchased outside of Japan. Read the basics here:
http://www.japan-guide.com/e/e2361.html

2) From Website, I understand the pass is valid for any JR including Shinkansen, Limited Express, Express rapid & local. But if the desination that I intend to go require me to take Limited express train, does I need to pass extra even I already purchase the 7 days Pass

No, as long as all the trains along the way are covered by the pass, you do not have to pay anything extra.

3)Any suggestion for Onsen between Maibara to Nagoya station.

I am unaware of any good onsen in that area. But I found the onsen at the Nagashima Spa Land the best "human made" onsen in all of Japan. It is not quite along the way though. A separate small side trip from Nagoya:
http://www.japan-guide.com/e/e3313.html