I'm being pitched on leading a new project - What questions should I ask about it?

I'm glad your instinct here isn't just to automatically say "yes"; that's an easy way to get burnt out. I have 2 trains of thoughts here:

1. Questions you can ask about the project
2. Question you should ask yourself

Questions About The Project

1. What's the deadline? How negotiable is it? - You want a farther out deadline that is ideally negotiable as well.
2. How many engineers do I get to lead on it? Or am I just expected to do it on my own? - The more resources you have, the better. If you're looking to grow into a tech lead, this could be a great opportunity!
3. How planned out is the project? Is there a well-defined product spec (and maybe tech spec as well), or is it just a fairly unorganized collection of rough thoughts? - Evaluating this one depends on you. High ambiguity is bad in a vacuum but good if you want to grow the skill of disambiguation.
4. What's the business impact and priority of the project? Why do we want to do this? - You want impact to be high obviously. If they're having trouble eloquently outlining the impact, that's a red flag.

Questions To Ask Yourself

1. How overwhelmed am I? - If it's a good amount, lean towards saying "No" or reprioritizing.
2. Among my current work, is there anything I don't like? - Maybe you can de-prioritize a project you don't like in exchange for picking up this new project.
3. What am I looking to learn/grow in? - It's possible the project can help you grow certain technical knowledge and behaviors.
4. Am I looking for a challenge right now or something more stable? - This is deeply connected to how you assess the ambiguity angle.