3 recent national studies show that the rent to value ratio is at an all time worse (what they really studied was cheaper to rent than buy, but there is strong correlation).  

Since you have no contract, there is nothing that says you do not have to pay if you are not satisfied, and your spreadsheet would seem to indicate you owe him the money.Similarly, since you paid him for previous work on the spreadsheet, and the spreadsheet shows you owe him for additional work, your argument is not very strong.

You just discriminated against them based on family status.It's good to have a heart-to-heart talk with them about what they can afford and see what they want to do.

Not at all.... the rent is based on the market rate, so if the units are all different...size/location/lay out/age etc etc... they would have different markets rates.

Based on your assessment, it feels like the only way for you to buy something now is to make a compromise that you don't want to make or to get out over your skis a bit more than you'd like. 20 years from now, it won't make much of a difference whether you bought something today or two years from now. 

They already have a track record (good or bad) with you, so you can decide based on payment history, inspections, etc.

I built this budget template based on my research but am only seeing bad investment returns / don't really know what I'm doing and would appreciate any feedback you all have to offer.

Then the tenant will rant a bunch of nonsense, have no evidence to back up his stories, and the judge will find in your favor based on the evidence provided by you.For future tenants, I highly recommend you read a book like mine or "Every Landlord's Legal Guide" by NOLO.

My mentors (both successful investors) swear by building strong relationships often saying  "It's all about who you know,".

This makes a strong partnership agreement critical to protect your interests.Legal and Tax Implications: Consult with a real estate attorney to formalize your agreement and ensure compliance with local laws.4.