Well at $600 for a RX 580 it may take you a year of 24/7 mining to even get your money back. If you ever want to sell the card to upgrade in the future, you will take a huge loss. If I were that desperate to start mining, I would rather buy 6-7 used HD 7850 2GB on eBay for ~$500-600. That will give you 108-125 MH/s on Ethash coins with a Dag <2GB like UBQ, EXP, PIRL, etc and also ~1170-1365 H/s Equihash and ~3000-3500 H/s on Cryptonight. Much better ROI and you can also trade those coins for ETH if you want.
I'm not desperate to ''start mining''. I just have some extra cash I want to keep rolling, since I don't have time to day trade. And I don't want to do day trade.
Where did you see 5-6 of 7850 giving you ~20 MH/s per card? As far as I know, maximum hashrate of that card is like ~12. And I don't think anyone would buy those off me even after ROI. It'll be hard to sell after. I think.