I have a 4-plex that is half STR and half LTR. 

Remove 2 first floor walls with engineered headers and added a half bath on the first floor.

I am currently under contract to buy SFH (3/1, 1058sf), with a half done garage conversion (450sf).

If your CF were to be cut in half on the original properties, you would most likely be able to increase your total CF by 5 times that of the new reduced CF. 

My wife and I are in the same pickle because some of these homes just 18 months ago were half the price but I guess if it’s gona make money then that’s all that matters right?

If the assessor really doesn't have any type of split- you can use the land value per the appraisal would be the next easiest option. 

You can most likely live for free or live for half of what it would cost for rent all while your tenants are paying down your mortgage, you are getting fat tax returns, and you have an asset that appreciates dramatically, especially when levered.

You don't want half motivation, you want full motivation and full attention.

I am now getting my first home and iwant to airbnb the bottom half of the home and fix it up nice to attract high quality travelers.

The price of the loan, utilities, insurance, and maybe half a year of property taxes or so.