The Solo 401k loan term is 5 years for general loans.The Solo 401k loan term can be more than 5 years not to exceed 30 years if used to purchase principal residence for you as trustee/participant of the Solo 401k.Solo 401k loan payments are made either monthly or quarterlyThe interest rate for a solo 401k loan  is either: A certificate deposit rate plus 2 percent or theprime rate plus 1 percent.Solo 401k Loan payments are fixed payments consisting of interest and principalSolo 401k loan rules do not allow for Interest only payments or principal payments only.The maximum Solo 401k loan amount is either 50% of account balance or maximum amount of $50K.– Example 1: Solo 401k balance is $50K; 50% of $50K = $25K (the Solo 401k maximum loan amount)– Example 2: Solo 401k balance is $150K; 50% of $150K = $75K; however, the maximum permitted Solo 401k loan amount is $50K– The minimum Solo 401k loan amount is $1,000.The Solo 401k rules require the following proper Solo 401k loan documentation:– Solo 401k Loan Agreement– Solo 401k Loan Application– Solo 401k Loan Payment Amortization Schedule

The drawbacks are twofold; the sale of the property and appreciation of the property.

If you are taking a new FHA loan there is going to be a new up-front MI that equal 1.75% of your loan amount and will be added to your principal balance and you will have a new monthly MI. 

The DSCR is the ratio of cash available for debt servicing to interest, principal and lease payments.

However, I have to make absolutely sure that the rent income will cover all of the monthly expenses including principal and interest.  

Just cover the Principal due, but because the full loan amount could be $420K, you can draw on it if needed.

Look into 203k rehab loans, I hear its great but there are drawbacks.

After the initial establishment of owner occupancy, nothing prevents you from moving out of the home and renting it, unless you want another FHA loan.The second link states: Principal Residency RequirementWhen you take out an FHA loan, it's based on the understanding that the home will be your primary residence.

Your principal and interest are not considered operating expenses for this calculation.Traditionally cap rate is used for commercial properties and not for SFH or 1-4 unit properties.

You don't know how to solve the entire problem for a seller/principal, however if you connect with people who do, you can get compensated for introducing the parties who can compete the deal.