Hi Roxy:
Welcome to the Cakewalk forums!
"If install Sonar 8, will it recognize my previous installation of sonar 6 and keep all the settings and folder locations etc?"
Yes. I believe it is when you start the new version of SONAR, that you will be asked if you want to import (or something like that) your settings from the older version of SONAR, select yes to transfer your settings from the older version of SONAR to the newer version.
"The software I'm about to buy has been previously registered, the seller said it would be completely functional, only I won't be able to get support from cakewalk. Is that true?"
Yes. The above is true.
"There also is a Sonar 8.5. I don't know what it has that Sonar 8 doesn't have, but would I eventually be able to upgrade to that if I installed Sonar 8?"
SONAR 8.5 is the most stable and fully featured version of SONAR before the SONAR X series of products and the SONAR newest series of products. SONAR 8.5.3 is so stable, fully featured, and customizable, that some users still use it rather than the newer versions of SONAR. SONAR 8.5 is a full upgrade version of SONAR, but at that time Cakewalk changed the naming convention, so it was called 8.5, instead of SONAR 9.
You will be able to install SONAR 8.5 if there is a version of SONAR 8 on your computer. SONAR 8.5 was a downloaded version that depended on some files from the SONAR 8 installation (unless you bought the SONAR 8.5 installation disks). So, if you have the SONAR 8.5 installation package (downloaded version), you can install 8.5 after you have installed SONAR 8. You will want to update to SONAR 8.5.3, after you install SONAR 8.5, if possible.
As, with SONAR 8, you will be asked if you want to import your settings from the older version of SONAR, when you start-up 8.5.
EDIT:
Oh yeah, if you use SONAR 8, you'll want to update that to 8.3.1, if you can. So the best version of SONAR 8 to use, is to update to 8.3.1. And the best version of SONAR 8.5 to use, is to update that to 8.5.3.