In my experience, I've come to the realization that I need to get pre-approved, rather than pre-qualified, in order to get someone (real estate agent) to show me around an area and start helping me find a property I'd like to invest in. 

You can build a great buyers list, and actually custom tailor the houses you go after based on the pre-qualified tenant buyers you have.

Another obstacle is the fact that the banks won't use the income from the property to qualify.

@Cameron PriceI agree with comments above that if you qualify for the Solo 401k plan I don't see any reason why SD IRA would be more beneficial (unless you have funds in a Roth IRA like Chris).

I am willing to sacrifice convenience – and also some money – if it has a big impact on my ability to qualify for a mortgage in 18 months.  

First of all, most flips don't qualify for cap gains.  

I know most of you would tell me I should be pre qualifying leads before actually wasting my time to go to the property, but I wanted the experience of walking through houses, learning to build rapport and improve my sales skills.

As of right now, we do not qualify for financing these large deals so it is a way for us to build assets/credit history. 

And I'm nervous about refi-ing the condo, renting it out, and buying a multi-unit bc I'm not sure if I would qualify for a mortgage for the multi-unit because our student loan balance raises our DTI. 

The holder of the debt.Do not charge fees, it's not a finance company.Interest is as agreed, you need to check usury laws for the state and stay under that rate, generally usury law will not apply in a commercial loan.A note is not a qualifying asset for an LLC as a capital contribution for compliance where capital may be required.Look up sham transactions as well, I have no idea what you're trying to accomplish.