Buying a certificate - what to spend and where

Solution 1:

I recommend GoDaddy for buying the certificates, good price.

As for linking to an ip address, I would not recommend that even if it's possible. You want to link the certificate to your domain name, and point the domain or sub domain at your ip address in DNS. This way if you change ISP's, you just update the new IP in the DNS and back in business, no need to adjust the Certificate.

Also for SBS make sure to watch closely which certificate. I believe the multiple domain ones are recommended for SBS so that Exchange, Webmail, and Exchange over HTTP will all work.

Solution 2:

StartCom provides free SSL certificates. The StartCom Certification Authority is supported by the most common web browsers.