My dad recently retired, so he no longer has a traditional income, and though he has decent savings, his credit is spotty at best. 

ideally we would like one check as a traditional transaction, but the idea of 1500 month cash flow doesnt turn us off.  

Going the traditional route (i.e. not using creative financing) and assuming you have a W2 income that will fly with the underwriters at your typical bank, I can give you two options to start with.1) More Costly - Buy primary residence home with a 3.5-20% down loan, buy investment property with 20-25% down loan.2) Less Costly - Buy fixer upper property with 3.5% down FHA loan, 'house hack' by doing renovations while living in it for one year (to satisfy FHA loan terms).

And/or I guess I'm trying to figure out how do I analyze this differently if I were to pay cash for the property or if I do traditional financing with say 20% down, then bank loan.

These are usually targeted at bridging a short term gap that is not well served by traditional loan products.

Banks or traditional loan you will usually be required to have a good credit and good ratio of debt to income.

Getting your real (net of inflation) returns to north of 4% like what the traditional financial planners tell you is the "safe" withdrawal rate does wonders for your expense multiple.  

That raises a red flag , unless he has a relative or won the lottery , I'd pass and hire a realtor to assist me. 

If you have no cash and just started your current job you will be hard pressed to find traditional funding.

@Bryan Otteson Yes a Traditional IRA can be transferred to a solo 401k plan, but a Roth IRA cannot (this is a Roth IRA restriction-see IRS Pub. 590).