Questions about FHA pre-qualifying for multi-family

[Xin-Ran Jiang]

Background: I been working on Airbnb arbitrage for 4 years now. I rent properties and rent them out, under agreement with owner.Went through pandemic, I feel my strategy is mature enough for me to move on to next stage....  I am planing on purchasing a 4 units multi-family in FL. I will keep one for myself and rent out the rest 3 of them, either long-term or short-term. I have 140k gross income last year, no other debts, first time home buyer. My plan is 3.5~4% downpayment to keep the most cash in hand.

I am ready to connect with lenders, but have no idea how to calculate the potential pre-qualified amount. More specifically, I'm not sure about: 

1.  I am expecting a boosting in gross income of this year (to ~350k). This is based on the future reservation of my rental business (I guess I can pull a proof of income from Airbnb). Still,  should I add-on this future income for pre-qualification?

2. Since I am targeting on 4 units multifamily with FHA loan, I assume ~75% rental income from addition units can count in. However, when there is no such specific property referred, how to determine the maximum qualify amount, so I can start shopping around?

The price range for multi-family could be so wide. Good deal could be from $300,000 to $1,300,000. What amount should I start with? This bothers me a lot. 

If you know how to solve this problem please share your expertise below!!!