Need help building hobby rig.
So I have been dabbling in cracking for some months now and have been thinking about buying/building a dedicated rig myself. I will have a windfall of about 2k in the coming weeks which I intend to spend on a hash-cracking rig. In addition to the 2k I could add another 2 or 3k, all I want is a base to start working from that I can upgrade as time progresses.

Appliances will of course be cracking but also some AI/Machine Learning related stuff. 

I have been reading and reading but have mostly encountered really expensive setups that are simply out of my reach. Especially the server cases have me lost, the Tian cases are all way too expensive so I don't know what to buy. 

As of now I have been looking into buying 4x secondhand 1080ti's (since they are  cost efficient and able to be snagged for as low as $250 but not set in stone of course). Am holding of on buying anything until I see the 3070 benchmarks though, if they offer better price/performance I will definitely buy 4x of them instead. 

For the rest I am quite lost as to what I should buy, a rack that could support up to 6 or 8 GPU's would be nice.

What would the more experienced people here suggest for a set-up that will have the following as base: 

GPU: 4x 1080ti or 4x 3070/3080. 
CPU: No idea what to buy here
RAM: 2400 or 3200 Mhz 32GB/64GB/128GB
SSD: 1 or 2 TB SSD? 
Server Rack: The elephant in the room, I have no idea what to buy. Are there even cheap racks fit for hash cracking?
PSU: 2x Corsair 850/1000 platinum, or one giant brick.
Extra fans: Not sure if needed per se. But this I will be able to work out. 

Help will be greatly appreciated! I have been looking for help on this for weeks and haven't had any success.

EDIT: would this case for example be feasible for a hash cracking rig?
Most of the case/MB-combination depends on the cooling of the GPUs. If you can use GPUs with radial coolers you can stack them close together. That is not possible with GPUs runnig axial coolers, cause they just heat up each other, even in an open case like the one you've posted.

One SSD with >250GB should be fine for usual hashcracking.

The RAM equal your GPU RAM in size (or a bit less).