Time to buy a duplex?

[Andrew Davidson]

I wanted to start investing long ago, but the time wasn't right. Since then I bought my own home (appraisal 265k, balance 217k). Last August, grandpa went into an assisted living community. He agreed (and his kids) to an owner finance on his home (offered $40k, down payment $8k. Fast forward, it's renting for $600 a month and I am ready for the next purchase. 

I know I can't use my rental income to qualify, but thinking about financing a duplex while grandpa is still carrying paper. Good idea?

Duplex $80k, credit score around 660. 30k liquid available. Grandpa is happy with arrangements.