Kenneth, sounds like you're still early in the process so don't let the naysayers scare you off of further due diligence. You've been given good lists of the red flags so dig deeper in those areas. Sounds like you already have that figured out.
Because there aren't many other small MF comps in the area the seller's financial records will be even more critical. Hope the cash payments will be verifiable, (checking account records and quality of bookkeeping). Don't be afraid to re-adjust the offer price if the seller can't verify the income to suit the HML. Don't be afraid to walk away if the due diligence reveals major flaws in the story, but also don't hesitate to make a lower offer price to make the (actual ARV) numbers work before you do walk away, if that's the case. I love the BP rental calculator for figuring out the price that does make a deal work. I love how Brandon teaches how to work backwards from the asking price to get to an offer price that works, using the calculator. Watch his webinar. Even if you do end up having to walk away if the numbers don't pan out, it's not a failure. It's actually a success because you didn't buy a bad deal! The biggest thing that will help you achieve long term success is the ability to walk away based on the numbers, not how many properties you acquire. The most important process in REI is the due diligence, so do it as an exercise even if the deal looks iffy. It's not time loss but a learning/practice activity. The more you do it the more efficient you become at it.
Are you actively involved in a local REIA? If so, getting additional feedback from someone who is a vetted investor and knows the area, (once you have your contract with an inspection clause) it will help you make sure you are asking all the right questions.
Wishing you success.