If you are only going to do bruteforce you might get away with a repurposed mining rig, but you should be aware that those will have low performance for pretty much everything other kind of attack and most of the other attacks will likely require much more RAM than your average mining rig has (recommendation is RAM >= VRAM).
I don't know how familiar you are with hashcracking in general and for what purpose you are cracking those hashes but I want to point out that bruteforce is usually the very last resort and generally not very useful compared to other attacks.
If you are positive the limitations outlined above don't apply to you and you will be able to handle cooling such an array of GPUs go ahead. Regarding the GPUs you are buying, have a look at the benchmark values, for example here: https://tutorials.technology/blog/08-Has...d-amd.html
Check the $/hashrate rate to decide on the devices you get. It's generally much better to go for less devices (less heat and power) if the rate is close or better (disregard AMD cards). And of course make sure the cooler/fan is appropriate for your cooling solution.
You have been warned.
I don't know how familiar you are with hashcracking in general and for what purpose you are cracking those hashes but I want to point out that bruteforce is usually the very last resort and generally not very useful compared to other attacks.
If you are positive the limitations outlined above don't apply to you and you will be able to handle cooling such an array of GPUs go ahead. Regarding the GPUs you are buying, have a look at the benchmark values, for example here: https://tutorials.technology/blog/08-Has...d-amd.html
Check the $/hashrate rate to decide on the devices you get. It's generally much better to go for less devices (less heat and power) if the rate is close or better (disregard AMD cards). And of course make sure the cooler/fan is appropriate for your cooling solution.
You have been warned.