If you're buying from a reputable rental company or corporation, then you have no need to worry. There's lots of documentation.
Buying a rental car from a reputable company makes good sense. You know they've been well-maintained and they're reasonably priced, for the most part.
Know the company you're buying from, ... I would opt for purchasing directly from a Hertz or Enterprise. But I'd be careful of vehicles that come from corporations. They can generate lots of mileage.