Who had what issues? Professionals or end users?
Windows XP and Vista can join and run on a 2012 domain. Although you don't want to anymore, of course you don't want any XP rigs around anymore, but "doesn't work with server 2012" is not a valid excuse....because they can. People that say they can't get XP to join a '12 domain are probably trying to run the easy peasy Connect wizard, or perhaps something more problematic is there like incorrect DNS settings. And for supporting XP clients there are one or two things you have to load on the '12 server, like SMB1 (not on by default).
When you say server 2012 can't do a workgroup, are you talkin about 2012 Essentials? That DCPromo wizard keeps launching? Yup like its predecessor, SBS, it's licensed to be a DC only....and you wan't a domain anyways...plus all the cool features of 2012 Essentials experience. Just do a new domain from scratch, I'd skip trying a migration for a network of just 4 rigs.
And check to see that they're on a fairly current version of Slowbooks..I mean...Quickbooks.