Welcome to the PB Forum AW Ranch. You ask "What due diligence items should I conduct prior to closing on the ranch?" . I assume you mean as to the lake and not as to the entire property.

Given the size of the purchase I agree with the comments by JHAP and jakeb. I would also gather some soil samples from the pond watershed for testing and send them off to A&M for analysis. Bob could help with that. That info would be good to have for management later as well. A pond assessment by Bob would be good for both management and due diligence. Also good to find out about the neighbors.