I'm trying to buy a 4 plex, and live in one of the units.The problem is that the rents are currently too low for the apartments to cash flow with all four units, and the rents would just barely cover the mortgage if I lived in one of the units.I could live there practically for free but what would I do when I wanted to move?

Prices are high, expenses are high, and rents are modest so the strategies that can get you enough to live on, to give some examples, might be student housing, higher risk cities with low house prices and poor economic fundamentals, or starting with buying a multi-million dollar portfolio requiring hundreds of thousands or a million+ of down payment. 

How low of a credit score should I accept?

Also he would want to know if you ever owned or bought investment properties, or just heard on a crazy website or from a guru tht you need to make a bunch of ridicules low ball offers.

Any recommendations for banks with low fees/complicated setups (setting up a Wells business bank account was basically hell) etc?

He wants his security deposit back so he can pay it to the next landlord to get a place to move, if it's low income, you gotta realize most don't have savings and he wasn't planning on moving when you bought it.     

If you focus solely on the rate you may give up great future deals trying to hold on to your currently low rates. 

Vacancy rates might be one of the major downsides here, in addition to low population/job growth.

I am looking to keep my loan costs as low as possible.  

Maybe get a low priced master lease afterwards allowing you to make good profit from subletting.