That should work fine. You have to do what works for you in the end. If you do enough google searching you should be able to find the power consumption of all your devices and the power supply you may need. There are many calculators out there. Also, there are connectors that allow you to use 2 power supplies in tandem. Make sure when you buy a power supply you know how many 6/8 pin PCI-E connectors you have on the power supply. It is common to only have 4 6/8pin connectors (each plug can work as a 6 or an 8 pin), meaning you would need a power supply for each 2 cards. This is of course assuming each card has a 6pin and an 8pin connector on it. I believe that is how the 280's work but you should research all of this to be certain. A few hours of research can save you a lot of regret.

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Middlecoin is a good choice, I use them and like them.  With a limited hash rate, you can probably do a little better by watching the coins (coinwarz, coinmarketcap, etc), mining when diff is low, selling when value is high.  More work, a little higher risk, but more rewards, if you have the time.

Good luck!