That's aweomse, I have just seen one, in mint shape for $3000. The guy that bought it loves it.
But, I would get it if you wanted to be a collector. This 400 is going to piss you off around every corner. For it to be a reliable daily rider, you'll need lots of work. Carb work, valve work, new tires, new brakes and cable replacements. I'm saying this just looking at a picture, but I have the idea it's been away for a LONG time. And judging by the nice condition of the seat, I don't think it was used too often.
So for a daily rider, not a great idea - But for a collector, it's a great choice.