We have located a good parcel of land at a fair price, only issue is it's currently zoned for residential and we are planning to develop a small, 8-unit apartment on it.

used an FHA loan with 3.5% down, lived in one unit, and rented the others.

For 2-4 units most owners don't keep track of all their expenses so that will be a hard one.

Some of the areas that we invest in have some clauses in their zoning forbidding more than 3 individuals unrelated by blood or marriage from occupying the same unit. 

Most investors take a look at the available rental units in a particular market,  determine if its a good or bad market based on DOM and either dismiss the market all together or replicate the housing units that are absorbed.

Where have you see multi-family units appreciating and increasing in rental cash flow?

How do you calculate the Returns in a multi-family unit?

Here’s how you can approach both issues:For rent comparables, since RentCast and Rentometer are giving you different results, the best way to narrow them down is by averaging the estimates and focusing on properties that closely match the one you’re analyzing in terms of size, number of units, and amenities.

For example in many jurisdictions I can STR units in a duplex but cannot STR an ADU (some jurisdictions will let you STR if you owner occupy).

It can sometimes happen even shortly after buying a unit - life forces change sometimes.