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Ours didn't break the bank - old Dell server for £75 and over time spent around £100 adding RAM upto its max of 96GB - we didn't need to add that much, but patiently got it from Ebay when we could find sticks at the right price.
Old but perfectly capable graphics cards can be had used for relatively small amounts of money (£20-50)
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I would look for a second hand i5 or i7 tower pc
at least 8m ram, ideally 16
SSD definitely, and twin monitors
for graphics you don't need lots of grunt, but you do want it to be zippy