When issues come up, and eventually they will, such as vacancies or unexpected maintenance issues, this is where we've earned our reputation.  

As a retired CPA, I have seen more than my share of bankrupted RE investors who were over leveraged with many properties.All the more reason to emphasize developing good due diligence practices, having some skin in the game and staying on top of legal requirements, maintenance, finance and tenant screening criteria.

Have you had tenant issues such as nuisance calls or maintenance irks?

The property had a large amount of deferred maintenance.

Is the payoff on decreased maintenance/standardization of units worth the upfront cost.

They can also be another item that will need maintenance / replacements if they break down. 

You need someone with boots on the ground to drive by the property regularly, inspect every 3-6 months, supervise maintenance, etc.

I've run the numbers on a bunch of properties using the following numbers:- vacancy 6%- Property tax 0.44% of list price per year- Insurance $250 / month- Capex 5%- Repairs/Maintenance 5%- Property Management 10%I've found a couple with a slight positive cash flow (no more than $50).

I agree with Caleb-save money for a down payment and to have money for repairs and maintenance. 

Please see more below:Solo 401k vs Self-directed IRA:A Solo 401k has several advantages as compared to a Self-Directed IRA including the following which specifically apply to your situation:Unlike a Self-directed IRA, you can have the account for the Solo 401k at a bank or brokerage that does not charge maintenance fees and where you will have checkbook control.Unlike a Self-directed IRA, if you use leverage (which must be non-recourse financing in either case) to acquire real estate with your Solo 401k the income will not be subject to Unrelated Debt Finance Income taxGeneral Considerations Re Investing Retirement Funds in Real Estate:1.