I'm thinking about getting a cointerra IV but I dont know if I can put in my own ATX psu incase their one breaks (as I have been told they tend to)
The PSUs are replaceable but you need to replace them with the same type, it is not an ATX PSU..
You can find spares on ebay from time to time and I suspect if you needed one you could get one from the forum as well..
With all of that said the Cointerra is quite dated so unless you bought one for peanuts (literally I mean tens of dollars) and had very cheap power it would not be as economical as a current generation system (SP20, S5, etc.)