You should have some of your portfolio in the public stock market, because it is a solid wealth builder over long periods of time.

Charging interest is a deliberate design, where the design is periodic systemic failure.

I would just average what their cost has been and prorate the last time period. 

Keep in mind though that most have a minimum investment of $50K-$100K with hold periods anywhere from 3-10 years (ish, this varies of course), so you'll be tying up a good chunk of your solo401k as most of the time it's advised that these types of deals be considered "illiquid."

Best of luck and keep us updated on how this goes. 

They want to do 30 days to close, 3 day option period (no option fee though) and 20k in earnest money.

I'm in the process of refinishing the hardwood and updating the bathroom, then just tackling some minor cosmetics before moving in.

There aren't really any formulas for this, your best bet might be to look at the pictures in rental comps other than that I would estimate the difference to be about $100 in rent for every $1000 of rental value if everything in a non-updated unit is clean, usable and looks reasonably nice.

Also do I have to occupy the house for a certain period of time?

We are in our option period to buy a duplex in fort worth, TX.