I'm waiting to hear back from a real estate lawyer, but the way it was explained to me, is that this is essentially party of the selling price ($150k), but the deal is structured to reduce the state transfer taxes based on the $135k sale price, defer capital gains income into the next year for the seller (frankly, I think the 15k would not be capital gains, but I'm not sure), and reduce our monthly mortgage payments over the year.  

I know they'd want their share of the cashflow rather than just earn a return on a loan. 

If they can't provide those documents then reduce your offer price to compensate for that value. 

Additionally the business must show income (can be a very small amount), and remember unless your business is making substantial income, this is not a good vehicle for contributions, as the limits will be set depending on your earnings. 

. -- Also it is a two family and I looked up the average rents on rentometer then reduced the rents by 100-200 bucks a month for each unit. 1 unit will be renting for $1500 and the other for $1250.

Average people earn money doing things they don’t love.

If it helps, I've been in my current job 3 1/2 years (and hate it), earn about 43k per year, I pay $900 monthly rent now and don't think I can afford to pay more than that without other source of income, also loan would be only myself and my credit is in the 690's.My priority now is quitting my 9-5 job, but this end of lease is making me question what is the right next step for me.Any advise, comments and guidance is greatly appreciated!!

My goal in real estate is to acquire a large MF property (at least 100 units) and earn $250K/yr passively.  

Not sure if a lender will work with you on that, but it would reduce your monthly payment and you might be able to pull some equity out to help with a down payment.Hope this helped.

I also want to add that I have no financial support other than what I earn (no wealthy family, trust funds, or inheritance) so I don't have anyone or anything to bail me out if things go south.