You saved yourself a lot of money and in so doing, learned a lot from the exercise and which you are able to share with us.

My initial offers are typically low ball, but those low ball offers make the math work to help us achieve our RE goals.

I came across a apartment complex in Dallas area and did my learning exercise of cap rate. 

I have received 7 replies. 5 of which have told me a ball park amount that they would accept as an offer. 

But as a lessee in the lease option scenario it is in your best interest to exercise that option (assuming nothing drastically changes in your life, economy, or property and the terms you signed were favorable). 

@Jenkins Ramon - All solid suggestions, but I don't yet have the experience to estimate rehab/rennovation or CapEx, I figure that's something that I'll have to read up a lot more on to get ball-park numbers on.

Does anyone know if in a lease option deal there is a way that I as the assigner of a lease option contract can guarantee that the option fee I receive from the tenant will be guaranteed to be credited towards the down payment of the house vs the purchase price of the house, should the tenant buyer exercise his option to purchase the home.

I think rigorous exercise is hard to beat, your school probably has a jujitsu or running club. 

I think most people have covered the accuracy of Zillow very well.From my perspective, I use ZEstimate as a ball park number when I need a quick estimate for any wholesale deals I am working on.

HUD & MLSGood Exercise