Make sure you allocate money for this in your spreadsheet (10% of gross income). 

Take Yale’s endowment—considered the gold standard for its exceptional performance; 10% of its investment portfolio is allocated to real estate.

The rule is that your purchase price should be allocated between land and building in the same ratio as the value of each component bears to the value of the property as a whole.So let's say the property is worth $100,000 with $60,000 of the value attributable to land and $40,000 attributable to building (you can use any reasonable method to determine this; one easy way used by many is the assessor's split, though there are other methods that could yield a better answer for you).You buy the property  at a huge discount for $10,000

I'm not sure if you have the ability to save money not allocated to your household expenses.

You can allocate expenses to multiple properties by allocating a portion of the expense to each property.

If we could do this, we could just allocate sewer costs based on water usage proportion. 

As you scale your business it will become increasingly more difficult to allocate your expenses to the proper rental or business if expenses are running through your personal account.  

If the allocation of income/loss is not based on the capital contribution percentages, it is best to get a professional and draft a  partnership agreement.

Most of the people use the value allocated to the property tax documents. 

You perform a 1031 exchange from the current asset into a bigger asset of the same type within the allocated window allowed by law. (45 days to identify replacement + 180 days to close)C.