The customer only pays 1 price that includes the price for rehab and purchase.

I would use a software management software that people could pay online or setup auto pay.

I would love to just say "hey you owe 50% towards taxes etc." but I don't know what determines the commercial tenant paying which expenses.

@James McGovernGeraci and Doss have standard docs - believe one site is called lighting docsI have a set from my own attorney as I want specific language included in the documents

Tenants pay for gas and electric, owner pays for water.

Take the remaining money and pay off the loan on a SFR rental we currently own.  

I'm planning to make additional payments to pay off the mortgage faster, and I'm unsure how to account for this in the ROI calculation.

You may need some other consultants, like geotech engineer (soils report), site lighting engineer, etc. 

The short version of that is if you can buy 4 houses financed, it will make more money than 1 free and clear house;   especially when taking into consideration all the benefits that come with each house - your tenant paying down your mortgage, 3.3% depreciation on 4 houses, market appreciation on 4 houses, plus the rental income on 4 units.  

The problem is that there constantly seems to be fees being charged to me an example of this is in the contract  there is an additional 100 dollars allocated to sewer and water that the tenant automatically pays every month with rent and anything outside of it should be billed to the customer (there is no specificed time when this will occur on the contract) but it appears to be billed to me and after speaking to IIP it's like they made up rule on the spot stating that after 6 months all of the additional utility fees would be billed to the customer and in the meantime I am being charged for it.