If you do be prepared to have a rate of 9.50% or higher which in most cases would be a higher payment than if you simply refinanced and lost the 2% rate.You may also have road blocks with a HELOC that a cash out refinance would not have due to cash in hand and one loan. 

-Type of deal/loan that would best help me to achieve my goals. 

The loan biz has followed this formula for decades and there is a good reason for it!

With an FHA loan I would easily be able to buy the land cash and have the down payment.

I should be able to qualify for a DSCR loan or conventional loan, but I see down the road that this could affect my DTI and make it difficult for me to get more mortgages in the future.I understand I could use a conventional investment loan and my parents could co-sign the loan to give me better terms, but does this remove the loan from my DTI?

Purchase price: $380,000 Used my VA loan to buy this new construction detached townhouse at 0% down to live in.

I've thought of 3 different approaches - 1. co-borrow...multiple parties on deed, entitled to % rent & depreciation based on investment, have "access" to property2. co-sign...multiple parties not on deed, negotiate % rent & depreciation based on investment, no legal access to property3. side-deal...parties gifts the down payment, operator must qualify for loan, all other details TBDAre there more approaches/options?

However, I would like the solo 401k, if possible, for (i) the $50k loan option & (ii) no taxes when using financing.

that amount will be challenging to get funding for due to the high cost to orginate a loan.

Turned out he forged several of the loan documents and was eventually caught and indicted for fraud.