PROS of a Solo 401(k) with no Custodian: 1) Freedom of checkbook control, ability to more easily move funds in and out of a bank account in the name of the Solo 401(k). 2) In some cases, lower fees. 3) Ability to more easily link your Solo 401(k) to bank account systems like rental property management systems, or if investing in real estate syndication platforms. 

Having just a couple properties for a short time isn't enough sample size to determine what an accurate estimate should be. 3.

Could you share how much upfront investment is typically required to determine whether a project is worth pursuing?

If there's a family member or friend in a similar situation, and you can both align on the initial plan (location, home type, price, usage/occupancy, etc) you can pretty easily get a co-ownership agreement in place and even a joint mortgage.The benefit here is it might open up your ability to buy a larger place in a nicer area which could lead to a faster rate of appreciation and overall better living environment. 

I recommend outlining your financial goals and definitely exploring work transfers to determine what suits you best.

From an investment standpoint this reminds me I believe of a Bill Cosby story line I believe.  

You may have more ability if you have any previous 401k's from previous employers?

@Collin Luckett  Just adding, if you have the ability to fund with your HELOC, it would be far cheaper than PML or Hard money. 

Be sure to consult with your CPA to determine if you can utilize these deductions based on your specific tax situation.

I recommend starting by contacting the county’s property tax office to determine if they are open to discussing or reviewing an appeal for property taxes.