As with any second hand commercial, tread very carefully....avoid anything thats ex rental, post office or high fleet use such as the utility companies.
If you buy from an auction, do remember they are there for a reason i.e their economic life is effectively over.
The engine in the Fiesta is likely to be the 1.4 dci unit - reasonably well proven but musn't blow any smoke and musn't have any fuel leaks around the cylinder head area.
John
Impossible to speculate Mick - it will have had loads of drivers (few, if any with a caring attitude) and will probably have been flat out for all of its life.
You'll also pay VAT, which won't apply with a private purchase.
John