The difference between the 400 and 500 series is the width. The 400s are a four foot wide baler and the 500s are a five foot wide baler.
The main up grade from a 7 series to an 8 series is the heavier chains and sprockets. You also could get the larger tires as an option.
There really are few major differences between the 6 series, 7 series and the 8 series. Mainly just a few options that became available. Also the monitors kept being up graded.
I would just look at condition of any of them and price. There are good deals out there right now on all hay equipment. The livestock guys are taking a beating with high feed prices.
I have bought some real good balers in the last six months in the low teens, real low teens. I have heard of some not breaking 10 K on sales but I did not see them to tell what kind of condition they where in. A baler that needs many repairs like belts, chains and sprockets can easily need a few grand spent on it. So just make sure of the condition. A higher priced one very well could be a better buy.
If the baler you are looking at is close to a JD dealer pay to have them do a baler inspection on it. These inspections would only cost about $175. They would list anything they see that would need repaired or shows excessive wear. JD has preprinted inspection forms that list all the things that should be checked.
One word of caution on any of the balers newer than a JD 535. The JD 566,567, and 568 all have the bale counter built into the monitor. So if the monitor has been repaired or changed the bale count will not be correct for that baler. So look at the belts, the chains/sprockets, pickup and the sides of the bale chamber. These are the wear areas and will give you an idea of how many bales the baler has actually baled.
I have seen guys send the monitor away to be repaired just to get it zeroed. Then they bale a year or two with it an then represent it as a low bale baler when they go to trade it or sell it.
So go on condition not bale counts.