I started mining end of 2013 with 4x 280x but I have been out for long time and now I see there are like 300 different mining rigs.
When I started there was only SHA 256 and Scrypt then X11 and now I have no idea how many..
What can I build/buy for $4000/5000 max?
I'm not going to provide an entire recipe for you, but here's some notes to get you started:
If you can reuse most of your old parts, and only need to shell out for new graphics cards (and perhaps PSUs), then you're probably in good shape. If you're building a new rig from scratch, I'd think quite a bit more carefully.
Also note that at this point, you have many more options available as far as powered risers go. The ribbon cables have mostly been dropped in favor of the USB-style riser cards (6-pin power or 4-pin molex is usually recommended over SATA-powered versions).
The typical GPUs people go for right now are the Vega 64/56, RX 580/570/480/470, GTX 1080ti, 1070ti, 1070, and 1060. (Yes, Nvidia GPUs are actually viable options for mining some of today's cryptocurrencies.)
Lower-end ones are viable (such as the 4GB RX 560 or 1050ti), but sacrifice rig "density". (Lower overall hashrate per rig -and therefore revenue- for the overhead costs required for the rig base [motherboard, CPU, RAM, etc.].)
The GTX 1080 is less popular, but that can sometimes make it easier to obtain (greater availability or can be purchased at less inflated prices).
Some of the typical choices for a *relatively* stable mining experience are Ethereum, Zcash, and Monero, but you could earn more (or less) by gambling on smaller coins, or mining a coin within the first few hours/days of its release. (As you probably remember, speculation is an essential part of being a miner.)
If your electricity costs are low enough, you could probably profit with your 280x cards mining some cryptonight coin (such as Electroneum or Monero) or equihash coin (such as Zcash).
Lastly, note that unlike CGminer, most well-performing miners will have some sort of integrated developer fee somewhere around 0.5% to 2%. Use "cracked" versions at your own risk. (Although, in my opinion, the developers deserve their cut, even if those of us who remember ckolivas aren't used to all this closed-source crap.)
