I think the biggest challenge will be finding a good example - there aren't many left and there are quite a few rough ones about. If you can find a tidy Ford Mondeo ST220 that's clearly been loved, it'll certainly be a bit more fun than your Mondeo Ghia X (provided you can put up with the harsher ride quality). A full service history is a good sign, as is premium tyres and tidy bodywork. An ST220 will never be a serious investment but, find a good one at the right price, and you shouldn't lose money.