It's a little bit of a red flag if the guy claims to know so little about it, unless there is good reason for his lack of knowledge. For example if he inherited it, or bought a piece of property with the tractor on it...he might lack info. But it could also mean he bought it at auction or it could even be stolen. As oldnslo mentioned, you can gain some year range info from the model. And the EPA tag is a great idea. The tractor will be at least as new as the tag. Wheels are dated also, but they can possibly be a year or two older than the tractor.
Issues with early Branson 3510's were minimal. They went through clutches fairly quickly if an operator was not careful. The clutch would not tolerate excess heat from slipping the clutch. Replacement clutches lasted much longer, but engaged more abruptly, hence they do not generate much heat. Tie rod ends were small and failed early, but there is an upgrade kit for this that solves that issue. The power-steering tanks were plastic and prone to failure in about 5 years. For about $95 you can replace the tank with a metal one. Occasionally a power-steering line will blow, and they are a pain to replace and you can only get them via Branson. There were a few other minor issues early on, but not much. The tractors are basically really solid units and with a little care will get a lot of work done.