Having been in a similar situation in the past, I discovered all sorts of nasties. If your PC is old, it's probably an IDE not a SATA drive, which means adaptors to get it to work with a new motherboard, etc etc etc.
To cut a long story short, unless you have an inside knowledge, buying a new or nearly new machine (system unit only) is probably a better bet.
I was looking at the fat end of £270 for a new system unit only to find that I could get a new laptop for £300 (Compaq), and that included free software deals which I would not be able to get on their own. The only reason I replaced the 2004 WinXP unit was because it had become unreliable in powering up.
To cut a long story short, unless you have an inside knowledge, buying a new or nearly new machine (system unit only) is probably a better bet.
I was looking at the fat end of £270 for a new system unit only to find that I could get a new laptop for £300 (Compaq), and that included free software deals which I would not be able to get on their own. The only reason I replaced the 2004 WinXP unit was because it had become unreliable in powering up.