With a budget, I think I would have started with just 1 RX 470/570 cheap, and then when it has earned itselves back, I would have brought one more, but keep in mind of your price a kWh. Denmark which is where I live, charges $0.32~~ kWh. Very very expensive, but lately I have earned lots.
PCI-aux? Not sure what you mean here, but I would go ahead and buy a PSU which can handle mining.
I agree, I will hold my currencies and see what happens next, I mean, this is future. Ethereum just have to fix the ongoing lag because of ICOs.
Investing in Bitcoin would be a good idea, as segwit2x soon will be deployed in the BTC blockchain, but remember to store your Bitcoins safely! On your PC/usb... Keeping your coins on exchanges is not a good idea - it could get hacked, etc.
If I buy more GPU, I would go for NVIDIA cards, GTX 1060/1070 as they arent affected by the dropping hashrate.
RE: Ethereum mining: DAG file grows, RX series hashrate decreasing