Wraparound Loans

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I am a newbie that plans on getting started int he Fall of '06 (currently a studetn at a University). I am trying to learn as much as possible about RE investing and have a question on seller financing:

Let's say I purchase a property for 100,000. 20,000 down with 80,000 loan @ 6.5%. I then sell the property with seller financing. Offer zero down and 8.5% interest with a sale price of 130,000. From my understanding I will make money on the interest as well as the 30,000 off the top. Correct?

Do investors ever have concerns about the DOS Clause? Also, doesn't the fact that you have a mortgage out hinder your ability to get another mortgage despite the fact that someone is virtually paying you rent?