I wouldn't buy a computer just for hash cracking as you could rent a gpu compute vps.

If you need to buy it, the more gpu power the better, since hashcat is optimised for cuda. At the moment the best consumer-grade gpu is the 4090, as it will give you the best hashrate. You can also find used 3090 from miners but be careful, since they can have been treated in a shitty way (eg. https://www.youtube.com/shorts/ZQqU8wf5N5I )

https://uk.pcpartpicker.com/list/FNtGRv 

You will need something like this. These are barebones for hash cracking (If you don't know what you are doing... I would suggest something less pricey than this)