Old, cheap PCs (like a refurbished Optiplex 7040 for example) are quite nice for this. Depending on where you're living you'll usually be able to find one for $200 of eBay, Amazon or any other marketplace.
Regarding the spec, I'd say
  • 8GB is a minimum, and even better if you can go with 16GB (from my own experience DDR3 is still ok)
  • Intel i5 is absolutely fine, i7 even better
  • for storage, you'll usually have to get it yourself, since most machines go on sale with cheap 256 GBs. I'd for a good 2To SSD here, to have some piece of mind. It'll usually cost around $200 as well, so you'll end up at around $400 total. If you're really on a budget a 1To SSD will do for now, but I really think the extra 1To is worth the price.
Avoid RasperryPis, they're not suitable for this task in the medium/long run.
Happy Node Running!