I don't think I would do it without making a related side project first where there's no time pressure and can pause anytime - and I don't really care if it takes 300h or 900h because it's a side project. I don't even have to finish it.
Can you charge for project instead of billable hours, or agree on a fixed number of hours up front? Would you be willing to work more hours than agreed on if you aren't making enough progress?
Are you willing to potentially fail?
Not a problem if you get paid for all your time, rates rather than lump sum seem sensible here.