Joining a startup is a high risk/high reward decision, and the biggest question you should have for them is where is the money coming from, and how long does the startup have until it needs more money given the current plan.

In addition to the salary cut, what are their expectations of a first employee? Will you be the only engineer, what are their expectations for hiring more staff?

Raising money is very hard, do these founders have an amazing network that they can utilize to raise cash? Have they successfully started companies in the past? If you were an investor would you invest in them?

Also keep in mind that everything is negotiable. Finding first employees is very hard, and as such you have a lot of negotiating power, especially if you're already employed and aren't in a rush to move.