Advice for 2nd rental property purchase

[Dave N.]

I purchased a duplex 2 years ago for 20% down / $560k on a 10 year arm at 2.45%.  432k still owed on the home.  Mortgage payment is $2500 per month.   Unit 1 generates $2300 per month.  I live in unit 2 but it could easily generate $1800-2000 a month.  Duplex will appraise around $650k.  

I've got my eye on a 2nd duplex thats $540k that will garner $4k a month in rent.   Ive got perfect credit but only have $40k that I could take out of pocket right now.    

Would love to get some suggestions on how to make the next move here.   Im a noob to this.