Build a diy pc with six r9 390 gpus.
There are a lot of builds on this site.
Since you have low power cost. The r9 390. Works for you.
I will build a PC myself. I will search a build guide that supports six Radeon R9 390X cards. In order to build the PC, I will follow the build guide on BCT. And I can put my HDD into the PC or even have internet connection. I hope I got it right and I thank you for your suggestion!
I think I can do it as described now but since there are several build guides for different number of GPUs, where does the magical number six comes from? Also, what does make Radeon R9 390X make different ( the optimal for my purpose ) than other GPUs?
R9 390 - NOT R9 390x - are low cost high hashrate.
R9 290 can match them though, tend to be a little lower cost, but are harder to find.
The only REAL difference between the 290 and the 390 is bios update - the actual GPU chip is THE SAME.
(edit) and it appears that many or all R9 390s had somewhat faster memory
Cooling varies with the specific model of card - both came in several versions from different manufacturers.
The "x" models have more shader cores, but due to memory limitations they do NOT mine any faster on Ethereum, and they tend to draw a little more power when mining Ethereum than the non-x models (not a factor for you) *AND* they tend to run a bit hotter, AND they tend to cost more.