Controversial opinion but learned through experience the hard way here, repeatedly proven, anyone that you feel "like lack of commitment from him could possibly influence our culture in a negative way", just pass (at least until you have product market fit or a repeatable business running). As soon as shit hits the fan, they will either leave or drop performance, and you will have to let them go. In your case, I thin…
Passion is important, and there's a huge spectrum between "passion as an excuse to get people to work many hours without compensating them" and the bill-by-the-hour contractor attitude. You don't want your first employees to be anywhere near the right of that spectrum. Basically, you want them to proudly wear the company logo t-shirt. But this is perfectly possible without driving yourself into a burnout.
While I agree with many commenters here that way too many startups work their team too hard for absolutely no benefit to anybody, I do think that employees of small startups can be expected to have said startup at top thing on their minds, bordering family etc.
This information is missing from the OP's question. There's a world of difference between an applicant going "oh, hm yeah I can do some work for you but only 3 days a week because I'm actually focused on my side project and I hope it becomes a proper startup someday" and "woa that sounds awesome, I'm all in! but I don't want to work more than 3 days because I've a couple of hobbies / open source projects / whatever that I like to keep time for". In the first scenario, the engineer might be writing hours for you, but actually thinking about their other thing at the same time. They won't be super productive, and they'd especially not be all too creative.
Like you, I'd never recommend that a startup hire someone who doesn't have the success of said startup at top of their mind (again, possibly bordering family matters). But I disagree with your suggestion that the amount of hours/days committed is a good indicator of that. Part-time is the future, folks, accept it, embrace it.