Drugs are prescribed based on a cost-benefit analysis. In general, the medical establishment is pretty conservative (there's little benefit to the doctor if your problem gets solved, but if they hurt you they're liable to get sued). In the usual case for amphetamines, the cost is the risk of side effects and the benefit is helping someone manage their ADHD. For you, the cost is the same but it sounds like the benefit is much bigger. So even by the standards of the risk-averse medical establishment, this sounds like a risk you should take.

You're an entrepreneur. A successful entrepreneur thinks and acts for themselves. This could be a good opportunity to practice being less scrupulous. Paul Graham on what makes founders successful:

Naughtiness

Though the most successful founders are usually good people, they tend to have a piratical gleam in their eye. They're not Goody Two-Shoes type good. Morally, they care about getting the big questions right, but not about observing proprieties. That's why I'd use the word naughty rather than evil. They delight in breaking rules, but not rules that matter. This quality may be redundant though; it may be implied by imagination.

Sam Altman of Loopt is one of the most successful alumni, so we asked him what question we could put on the Y Combinator application that would help us discover more people like him. He said to ask about a time when they'd hacked something to their advantage—hacked in the sense of beating the system, not breaking into computers. It has become one of the questions we pay most attention to when judging applications.

I'd recommend avoiding Adderall as a first option. I've heard stories of people whose focus got worse over time as tolerance to the drug's effects developed.

Modafinil, on the other hand, is a focus wonder drug. It's widely used in the nootropics community and bad experiences are quite rare. (/r/nootropics admin: "I just want to remind everyone that this is a subreddit for discussing all nootropics, not just beating modafinil to death.")

The legal risks involved with Modafinil seem pretty low. Check out Gwern's discussion.

My conclusion is that buying some Modafinil and trying it once could be really valuable, if only for comfort zone expansion and value of information. I have very little doubt that this is the right choice for you. Check out Gwern's discussion of suppliers. (Lying to your doctor is another option if you really want to practice being naughty.)