Banks and alternative lenders (non bank) are a possibility.

There may also be other creative financing alternatives available, such seller financing, partnerships, private money lenders, or utilizing the equity in your current home.Do extensive due diligence to evaluate a multi-family property's finances, condition, rental prospects, and market worth before making an offer.

Considering this constraint, I am contemplating an alternative approach: constructing a fourplex on a suitable piece of land.With a capital investment of 25%, I am curious about the financial viability of this strategy.

As an alternative, would you feel confident retiring on $5k/$10k/or $15k/mo of income that increased about the same as inflation every year?

There are some alternative owner-occupied options to SBA if you don't qualify for it, however it may not be the rate you are looking for. 

Real estate moguls, precious metals enthusiasts, and alternative investment seekers, an SDIRA is your ticket to diversification.Your SDIRA Investment Menu- *Self-Directed 401(k):** Perfect for the solo entrepreneur.

Diversify the portfolio in the markets, bonds, and some alternatives like real estate.

Just an alternative thought, depending on the state you are located in, would it make sense to open an LLC for the management of your rentals? 

You should bring the lender your 'business plan' for your rad therapy business.

Evaluate your savings, income, and outgoings; moreover, educate yourself about mortgage alternatives and programs that help with a down payment.