I had a Mondeo V6, not a ST220, but the problems/areas to look out for are the same.
Basically the ST220 is a fantastic Car - fast and "refined" just make sure the engine one the you buy still "whines" when you drive it. You will know what i mean when you hear it.
You might want to replace the water pump for one with a metal impeller, or failing that replace the one in the car with a new one as soon as you get it - if the water pump fails, your looking at at least £1500 to put it back together.
Failing that, just listen for the usual noises when you test drive it - bangs from the suspension usually indicate worn wishbone and anti rill bar bushes, but these problems are common across all mondeos of that era. You may even find that you end up replacing the nearside front wishbone at least every MOT!
MPG wise, driven "sensibly" you will get around 25mpg around town, thrash it (you have to do that sometimes) you get between 10-15mpg but cruising on the motorway.....you get about 30-40. Just stick it in 5th and leave it there - and it will go from 35-120 without complaining, and it will do it quite quickly too.
I can't think of anything else right now, but if i do i will come back - oh, don't lower it. I lowered mine 50mm. By the time it had "settled" it was 75mm lower and I got it stuck in an NCP Multi storey Car Park. The bloke in the car behind, and the car behind him too were mightily pissed off having to reverse down the up ramps....
