Guys around here will pull those metering boxes off the planter and take them to a guy (dealer, etc) who has a stand that check the seed drop. If accuracy of drop is important to you, this should be something to consider.  Since you don't own this planter, you will have to know how much you are willing to spend, if they tell you that you need significant upgrade.

I see better stand where guys take the John Deere meter systems and upgrade with Precision Plant parts.  You didn't mention if this is still all John Deere parts, or if the owner has Precision Plant parts in it.  If you buy the planter, I suggest looking into this.