For me personally, when buying a used L/G tractor, I try to find something under 500 hours as a general rule.....just as added insurance. The previous owner could have treated it like a baby, or like a mule.....and that's nearly 1200 hours of use you'll never know which it was.
Having said that, the upper X series JD's are really nice machines, and built to last.
It's encouraging the dealer let you take it home to try it out....so they have some confidence it's working well. Did they do a servicing on it? Check the filters to see if new. Check the fluids to see if they are fresh, and clean.
They probably paid 3,000-3500 on the trade in, so start your negotiation appropriately.....enough to give them some profit, but to save yourself some $$. If they wont budge much on price, I'd ask for a 30 day warranty just to cover yourself.....
Good Luck!