I found a duplex on Zillow and the prior purchase price was 96k assuming they had a 80% mortgage they should owe about 71k on the property which is close to the asking price of 74k.

This is just the beginning of my research and I understand a lot needs to be looked into/evaluated prior to purchasing the land. 

There is no way to fully mitigate this, but be sure to have detailed conversations with them prior to getting them in a deal.

Super new to BP but I've been scouring the site for info about using retirement funds to invest, and their overall value compared to REI.

I agree with Brian's recommendation that when you compare SD IRA and Truly Self-Directed Solo 401k plan you will be better off with the second choice (by far!!!).

Anyone who make more than $400 net profit at the end of the year is taxed on it even though this might be their only income.With this in mind, can Michael N say "I am a business owner" (as a sole proprietor) prior to starting his trip to Kansas and start deducting all his business-related expenses from this moment on?

Reverse exchanges can very handy but they are relatively expensive and the cost compared to simply paying the tax on $20K or so usually doesn't justify it.Your scenarios all look fine.  

The form requires the appraiser to input comparable sales and also active rent comps.

I think the best luck I have found is to look on craigslist or check what a comparable property rents for on Zillow in same area.

Now the seller had already been working on getting the C of O prior to listing the house.