Would i be mad to throw any money at this machine? Yes.
Possible reason for fan running at Concorde speed is that the internals will be clogged up with dust. Remove all sources of power and take off one side of the case. With a new small paint brush, brush off the dust from everywhere, including the cooling fins on the CPU, while holding the nozzle of your vacuum cleaner near-by. Just be careful when you do this so take your time.
From the back of the PC take the right hand side panel off.