Enjoy everyone !

New generation AMD 4xx/5xx GPU's are currently out of stock at my local. Sellers are only willing to sell these new GPU's %20-%30 more than its price.
In deed there are some 280x,290x and HD7970's stand on market.

prices:

(New)RX470 $335
(New)RX480 4G $330
(New)RX480 8G $350

(Used)RX470 $250
(Used)RX480 $250-$300
(Used)R9 280X $125
(Used)R9 290X $160
(Used)HD7970 $140

What would be the most logical and budget friendly choice to dig Zcash ?

Bonus: I m also curious about 280X's TDP. I am a freshly new comer to the mining community and just want to test drive BIOS Modding, undervolting, efficiency and so on. If i can persuade myself that i was succeed, i may invest bigger.
I have a 5 Pci-e slot Gigabayte AM3+MOBO and i plan to make 5 GPU rig with it. But question is how much power needs to be supplied for these hunger 280X's. Can i supply 5x 280X with a Crosair RM1000i after undervolting ?

I will be much appreciated with your directions.