You got a steal. That machine new is around $1800.00.
Don't know about the warranty part. I thought Janome warranted their machines (yes, even used) the same. Guess you will have to check that out. Make sure you take it in to them for any problems before the 6 months is up. Use that machine like nobody's business between now and then. Use everything on it and don't be afraid to try all the feet and stitches. Six months will go by fast and you won't want to miss out. Then, if you don't like it, you will still be able to get your money back when you sell it.
As far as the Janome that is chipping, button losing design paint, etc.
My quilting teacher has had hers for about 6 years. She has sewn over 50 - 75 quilts on it, fabric bowls, and a gazillion other things. Still looks new. No chips, no lost button design, nadda. I have had mine for about three to four years now (I lost count), and still looks new. We both treat our machines like babies, are not rough on them, do not wear lotions or eat greasy, sticky foods around them. I am wondering if you wear a lotion that is robbing the button of its design. Anyway, should have held up much better than what you describe.