1-Does SAP Adapter with version 6.5 exist??
Yes it does.
2-Does this version 6.5 correspond to Integration server version or is different from that?
6.5 is the SAP Adapter version.
3-Could some one please tell the latest version of SAP adapter and the procedure to download it.
The latest version of SAP Adapter is 6.5 and you can download it from Advantage website, this website also contain the documentation on how to install and configure it.
P.S :You need to be authorized by your company to download the webMethods components from advantage site.