You want a facelifted 2003> car as the spec of all models improved - anything Zetec and above gained a trip computer, 6 speed gearbox, proper climate control (early high spec cars had a CC system, but it's rubbish) and cruise control as standard. This should be easily acheiveable for your budget - I sold my 2002 Zetec with 53k on the clock for £5k almost a year ago so I can't see it being a problem.
Zetec-S had different bumpers and 18" wheels, but otherwise was a Zetec. Personally I don't think the 18" work well on anything other than the ST, and the tyres are pretty expensive which will soon bump your running costs up.
Ghia adds a bit of chrome and wood, softer suspension than the Zetec models, Sony CD player.
Ghia X is as Ghia plus heated leather, wooden steering wheel, folding mirrors and 17" wheels.
Avoid the LX as they have different seats with less support on the base.
If I were you I'd go for a facelift Zetec or a Ghia. If you can find a Ghia X then fine, but don't pay a premium. I'd take a lower trimmed facelift car over a pre-facelift Ghia X.