Vermeer, for the most part, has always made very respectable round balers. That being said, you want to avoid the C series, because (like all balers of that era) they were closed throat. Which makes them much, much more picky to use, finicky, hard to start the core rolling, etc. The J series was 100X the baler that the C series was. The F and G series were the first open throat Vermeers, and quite capable balers. A little odd in some respects, and not quite as friendly as later balers, but still decent. I have a 504G at our Southern farm and get along fine with it at that farm: We don't do nearly as much hay there as at our Northern farm, so I get along fine with it. At our Northern farm we now have newer Deeres. You definitely don't want any Vermeer earlier than the F/G series. The same applies to any other closed-throat, regardless of make, like the Deere 410/510 (also closed throat nightmares). The Vermeer series I and later were great balers: foolproof and robust. You can't go too wrong with that I or J series. They also had the four-digit series (like the 5410) which were also good balers, but made to be economy, lower HP balers. The 504/605 series is more of the commercial series, while the 5400 and 5410 were more aimed for the hobbiest doing only a couple hundred bales a year at most. Both good balers, but the three digit series is more robust.
Between the two you're looking at, the J is the better, more desirable, more user-friendly, and less finicky baler by a looooong margin. Well worth the extra $700 (assuming they're both in comparable condition). I'm not necessarily saying that you should definitely go with the J. But I will say that you should definitely avoid the C. And again: That's not because the C series was bad compared to other round balers from the same time period. But rather because the open throat balers that came out a few years later are 100X better than the older closed-throat balers - regardless of make.