HELOC to fund Multi-family Downpayment?

[Kayla Eresman]

Hey everyone, I'm fairly new in the world of real estate investing and I could use some advice. 

We are looking to expand our real estate portfolio with either a 4 or 6 plex, or a commercial building. We have a single door rental worth about $320,00 that will be paid off on June 1, a half duplex that is worth 260,00 with a $120,000 mortgage and our primary residence is worth $650,00 and has no mortgage. We also have $350,000 in invested cash earning around 6%. What's the play here? Should we front the downpayment from our investments or do a HELOC against the house worth $320,000? Or is there a different avenue that we haven't explored?

Thanks in advance for any help!