I am currently using X3 Producer, but I have versions going back to 8.5 (maybe 8).  Although I do not run the earlier versions I do still use vsts from the older versions.  I will shortly need to re-install Sonar - hard drive giving warnings.  My question is - should I re-install everything starting from 8/8.5, or should I install X3 and then cherry pick the things I am using?
 
Cheers
 
Grif

Install X3 completely and use advanced install to add missing plug-ins.

are you sure you need/want to do a re-install ?
maybe the hard-drive warning could be solved without re-installing....I would try to avoid re-installs as much as possible (not that a re-install has to be a prob, but personally I'm better off since I stopped doing that)
 

I just had to do a re install after Windows 8 asked me to perform a "refresh" It put everything back including Classic shell but all my software was gone! 
I always install a bare bones version of 8.5 to grab the stuff that I don't get with X3 studio. I uncheck 70% of the boxes.  It doesn't take up much space and only takes a few minutes to install. I don't even run it other than to enter the activation code. 
 
Best order is to install your audio interface and any other drivers like for USB keyboards or Drums. Then install Sonar form oldest to newest. 

I upgraded my pc recently and did a fresh install of X3e and in the process lost a lot of my old plugins (going back as as far as Sonar 8). I reinstalled my favourites from disc but a lot of them no longer worked with the more recent version and seemed to cause heaps of issues with X3. After hours of trying to make things work I surrendered and discovered that a) the headache wasn't worth it and b) that mostly the newer plugins are much better anyway. Not particularly helpful to you but just wanted to share my experience.