SEPIRA is a lot easier to set up than a solo 401k but also has lower limits depending on your income (and I seem to remember SIMPLE IRAs are a decent alternative to SEP but I don't remember the reasons). Honestly, I wouldn't worry too much about withdrawal regulations as they stand now because legislation can change in the future but there will always be loopholes to access your funds, including rolling over your 401k into an IRA once your business is finished. Just try to keep a good mix of IRA/401k/taxable funds and you'll be fine. Whitecoatinvestor has a good blog that applies a lot to entrepreneurs even if you're not a doctor, so I'd check that out if you want some extra fun reading.