Have they rented it through an agent in the past? If so, there might be a past disclosure. Also, they might be able to check with their insurance company for claims due to damage. The A/C, furnace and hot water heater should have been maintained by specific companies. They should have records too. A thorough inspection should tell you of the approximate age of the roof and appliances and systems as well as structural problems that are visible. If you do buy, I highly recommend a Home Warranty to protect you from unpleasant repair surprises. Your agent should be able to provide that information for you as well as reviewing comparable home sales with you to determine a "reasonable" offer. The fact that they appear not to have records does not make the purchase an "as is" home. If there are problems from inspection, especially structural, radon, or other large glaring problems, the seller should be willing to repair or credit you for repair unless they have declared that they are selling "as is."
Again, your best course of action is to work with a Realtor who has experience with this to help with troubleshooting for information, inspections, repairs, negotiations, and market analysis. If you buy without a lot of this information, you won't know until after you buy if you have a "diamond in the rough" or a "money pit." Not a good way to find out!