Although I conducted a basic septic inspection during the closing, and the system is in good working condition, it still failed the STR inspection.

If the house were in perfect condition, it would be worth around 250-275k but as of now, selling isn't an option because we may use that property in the future for our business. 

There have been times when (left in such good condition and clean)  I've refunded when they left early.

7 - What are the ground soil conditions?

Never built in Houston before so take my feedback with a grain of salt but I'd put money on it being impossible to build a duplex from scratch for $125 a foot (including everything from drawings, engineering, permit fees, general conditions, labor, material, landscaping).Out here in AZ the builder cost is going to be $200 a foot at best, but add 20% on top of that for the client cost.

However, with the rates and conditions of any triplexes and fourplexes we could find (we did find one, but it's in the Pendleton Heights suburb of KCMO, so we were worried about it being too dangerous of an area), we were wondering if it's worth it to buy a duplex and rent out the other half to help pay the mortgage, or if it's more worthwhile to rent an apartment and try to save up to buy a residential or commercial MFH later?

What condition in the house in?

Overall, this seems like a strong deal, but make sure to factor in potential surprises or changes in market conditions.

Safe rent after rehab is $900 per month, maybe higher if I get lucky and/or rehab it to a very nice condition.  

Quick question on those who have done seller financing:- Buying a 0.3 acre downtown fort lauderdale multi family, negotiated an around 1M price w lot of development rights, double lot. seller got it way back early 90s for pennies- got good credit 800+, w2 job etc put 40% down and mortgage bank offered 6.62% 30yr fixed (CF negative as this would make current rent roll not too much compared to the PITI)- property is in ok condition, needs 20-30k repairs which seller OKd, and all 3 tenants are month to month, pay bit under market but also ok- seller would like to do seller financing, interest only at 5.75%, 30yr amortization at 7yr balloon (CF would be positive) - id be paying some principal as well, just to grow some equity- buying this deal for the future development of the area (las olas, kushner broward crossing) so not really worried even if we overpay for it now, but never done creative finance as i always relied on a strong W2 job to get good loans but obv im used to 3% 30yr rates from covid times not this 6+ environment..Goal is to hold the property 3-5yrs, and then actually develop it or sell half the land / refinance, pull out HELOC if it appreciates.Questions:1) What does he know that i dont?