Ask HN: Vetting a Biz Dev Cofounder?

Hi HN, I have a potential opportunity to join a startup. It's a one person company, first time entrepreneur, self funded, with 15 years experience in the space. Founder claims top of funnel for first 1000-5000 users is seeded. Idea seems solid to me, working prototype is out, founder is clearly invested (2+ yrs and $$). Founder plans to exit in 3-5 after a VC raise in 6-12 mo.

What questions should I ask, or other diligence can I do on my end, before committing? Is there anything specific to watch out for? Are any potential questions somewhat undecidable, and if so which ones and to what degree and why?

Questions I am considering: - talk to the pre-seeded top of funnel prospects - talk to the interested VC investor - ask for personal references - talk to 5-10 prospects founder doesnt yet know to validate value prop - talk to the existing contractor team

Thanks in advance for advice / suggestions!