"offered to let me leverage his FHA"I am no legal expert, but this sounds like one of two things: 1) tip-toeing around the edges of mortgage fraud or 2) simply a bad idea. 

For example, If you invested into an apartment complex that was bought at $2,000,000, had a $200,000 NOI (Net Operating Income), had 100 units, was 100% occupied, and was bought at a 10% Cap Rate with just $25,000, then at the end of 5 years, lets say the property did great and was sold at a 8% cap rate, you would make a 188% ROI (Return on Investment) giving you back the $25,000 that you invested plus an additional $47,000 that you got for simply putting in the investment and watching it come into your bank account!

Simply because 75% or more of the movement is of the wheeled variety.

Simply because he thought that he could get more. 

For you simply studying might be the trick.

I am simply putting it out there to get some constructive feedback like others have offered.

Unless houses are very very cheap I assume you were simply missing some real comps.

I think simply sitting down with the seller and discussing what they need would be the place to start.

On the other hand, the property may appreciate in those five years, giving me "free" equity beyond simply what I paid into it.

If it were me, I would rip out all the edge tiles and replace them, simply from a momentum and finished result aspect.Good luck!