Only you can decide whether you want to pay more than Dave & Frank's system. I feel like right now brokered parks (that I see the listing flyers for) are trading at very high prices (like about 2x) and I'm sitting out for the time being. You make most of your money on the front end when you "buy smart." The annual expected return is compensation for your aggro and your return on capital. You can calculate the return on capital after debt service given the parameters you stated, but your time is worth something too. What kind of return is "worth it" to you? Can you do better? Can you do the same with less risk somewhere else? There is an opportunity cost to waiting, and so on.
Only you can evaluate the risk / reward profile and compare it to the aggro and determine if it's "worth it."
To specifically address your question, 30-year is the longest amort that you will see, and this is favorable to you. If it is not "callable" (i.e. not a 10-year balloon) this is favorable. If it is not prepayable this is disfavorable to you. 15% down is very favorable if you expect success (and what investor doesn't?), it is more than 2x higher leverage than 30% down. But, high leverage comes with risk, especially in a "frothy" market.
6% is reasonably low, by historical standards. Is this rate guaranteed for 30 years? This is favorable. Could you do better borrowing money?