What's missing from your post is the role handled by the server.
Is it just a file server for the workstations to store/share CS5 documents?
Is it a directory server that stores all the user's accounts?
Is it a home directory server that stores each user's home directory data?
That (and probably more) will make a significant difference in your plans.
If the server is just acting as a file server and not handling any other client services then it doesn't matter what the workstations are using - 10.6.x, 10.7.x, 10.8.x, heck, even Windows, or Linux, or Solaris... it doesn't matter in the case of file sharing.
If the server is running directory services and/or home directories, then you have a bigger issue. It's not that the XServer won't work with newer machines (it will), but that you have a larger migration plan to implement if you want/need to replace the server. Personally I'd be scared to run directory services and/or home directories with a single server - that single point of failure could bring your entire organization grinding to a halt in an instant.
In either case, the choice of OS platform for the workstations is largely separate from that of the server. I'm not even sure that the current generation of Mac Minis will run 10.6, anyway, so trying to unify everyone on 10.6 may be a moot point.
As for option 3 - a memory upgrade should be based on client need - are the users OK with the memory they have (which seems awfully small to me... I guess they're not running big CS5 jobs, or at least aren't complaining about performance). In either case, those prices seem high, even for the UK - right now, an 8GB upgrade in the US should cost about $100 (depending on the specific model), so I'd shop around.