Old thread...
Old Monarchs (and any other lathe) can only be as accurate as how much wear they have accumulated over the years. Exact same situation with the Jet lathe, and any other lathe. Pretty much any lathe that can accept change gears can make metric threads. It is not about the charts attached to the lathe, it is about the savvy of the operator. Any used lathe has wear and issues, and sometimes new lathes do, too. All lathes can be improved, if you know how and it is worth your time, effort, and money. Be realistic when pondering choices, and try to avoid pipe dreams.
Lots of very nice work comes off of worn out, loosey goosey equipment. Learn the skills in the time you would otherwise be drooling over ads for new and used machines.