NO. It's not okay to take equity without a written agreement. Established company, new company, or possible future company, it's still the same - no paper, no work.
You need a contract before doing any work. It is immaterial whether it's for equity or cash.
Since there is no company yet, the only contract you can get is from the one thinking about creating the company. You need to assess the risks of relying on his business plan, which means you need to see that plan. After all, by doing equity work you are "investing" in that business plan. Don't do equity work based on a business plan that you wouldn't invest cash in.
As this is still the idea for a business, the contract needs to limit how much work you do when. Have clauses that limits how many hours you invest in the company-to-be before it's formed, and what kind of output and time investment is expected of you at what stages of the company's progress through the business plan. Set limits, and enforce them. If you were investing cash in the business, you wouldn't keep investing your cash forever, don't invest your labor forever either.
You should also include clauses that control who "owns" your production. Until the company exists, and is able to compensate you a minimum amount, you should probably keep all copyrights for yourself. If the company never happens, maybe your work can be finished as a solo project, or re-purposed for some other business. If the copyrights don't belong to you, you can't and all your work is gone in a breath of air. Working for equity is a gamble, but that doesn't mean you can't hedge your bet.
If the other party doesn't "understand" your insistence on a contract, offer them equity in your company if they will supply a new computer, pay the monthly utility bills, obtain the development software you need, and cover the costs of your data connections, until you have time to find a lawyer and investigate an agreement for the same.