Get financials of tenant and payment history to make a decision.

Go to your local banks and credit unions.

When I was in college I was pretty irresponsible with my credit card.

There are not as many comps and it really depends on what is going on in the business world (tax credit wise, etc. ) to determine value unlike residential space where there is a general rule of thumb for the hot and off-peak market as well as a ton of comps for value. 

I have spent the last year working two jobs to save money and at the same time improve my credit.

Do you do background check and credit check also?

If so then you may be able to get 90% loan to cost depending on experience, credit score, loan amount etc...

My credit score is around 750, and I got pre-approved for 330K at 4.5% back in May.

You can also have your prospective tenants pay for Credit/background checks via Cozy, as well.

i would go with former tenant, however I would still have them do a credit/background check (at their expense using something like SmartMove).