I'm especially interested in a few small apartment buildings (10-20 units), but also some less-maintained single families.  

This is our first mobile home park purchase and we are doing due diligence following the workbook from boot camp.When we asked the seller for a copy of the survey and environmental study, the sellers agent said they did not do it when they purchased the park a few years ago and they also did not get it from the previous owner.Some agent said these two items are not required by bank so it is not necessary.

To purchase a $320K property, you would need to have a minimum of about $150K in the plan.As @Darren Eadynoted, any future expenses associated with the property would need to come from the 401(k), so one always wants to be conservative when maintaining semi-liquid reserves within the plan to cover any unexpected costs.That said, the ability to leverage your tax-sheltered retirement dollars into a secure asset such as real estate, and receive a higher cash-on-cash return for your invested 401(k) funds is a great strategy when approached the right way.  

After hearing about a local Real Estate Investor group in my area and attending a few meetings I've been studying diligently on Real Estate investing and this has been my main focus for a few months.

I was exposed to real estate at an early age and after studying business at the University of Michigan and practicing law for a while, I found my way back to what I really enjoy doing. 

They never missed a payment and always kept the house well maintained.

A real estate market analysis is a study of current market values of homes, comparable to yours, that helps you determine the market value of your house.

Many Walmarts have a bank in them and it's free to open a savings account there and no fees if you maintain a $200 balance and keep the number of outgoing transfers low; they also have a 24 hour drop box.

I’ve looked at multifamily buildings in my area that were well maintained where the asking price is not far off from what the owner paid in the 80’s. 

It will probably maintain the value, but cash flow will not be good, probably not the best investment.