I think you should not, because you're trading time which is your most valuable asset, and (statistically) you're not good at picking the right startup (read it as "you're not guaranteed to be good", I don't want to say that you're necessarily bad).
To elaborate. An angel investor has a lot of money, say a spare $200k, so she can invest $20k in 10 startups. If one of these does 20x in, say, 5-10 years, she doubled the money which is good.
You, instead, will only bet on one startup. From a purely monetary perspective, you shouldn't go too early stage to increase your prob of success. At that point, might as well work for a late-stage startup that offers cash + equity.
In your situation, I'd find a cofounder and a project, that can either become a startup, or even just grow to a passive source of income. If you arrive to a good amount of money, then you can think investing.
If you decide to go for it anyway, from a legal point of view I don't think you need anything. The easiest thing is that you'll become an advisor for a small percentage (I assume of very cheap stock options). Keep in mind that it will take several years before you these options may become stocks (company goes public) and there are many ways for the company to silently cut you out of the pie if they sell or exit in any other way.