General differences are as follows:
Post Mid 2004 Models:
Chrome exterior styling
6 speed gearbox
slight improved brakes
Sony CD player
Improved interior plastics
I'm sure there are some other things but they are the main things.
If you're not a fan like me of the chrome exterior, then if you look round you can pick up a part facelift, it will need to be a mid 2004 model, like mine, you'll find it has all of the facelift features apart from the chrome and better brakes.
The ST220 Duratec Engine is chain driven.
It will run just fine on standard unleaded and I should think Ford would have done most of their testing on standard, super etc is just a bonus if your wallet can hack it!
There are some various problems I've heard of, such as sagging rear bumper which happens to post 2004 facelift models... ford updates the rear bumper slightly and it has foam supports which eventually give a little leaving a visible gap between the boot/rear light clusters and the actual bumper (there is a fix for this in the tech forums). The only thing the Mondeo is prone to is the rear bushes going, but to be honest I'm not aware of anything else major with them.
Just be aware from my experience, whilst xenons are nice to have, if they go worng its hits your wallet hard... £90 per bulb from ford (can get them online for £45) and about £250 for the module attached to the rear of the headlight unit... one of mine went which is basically a chunk of metal with electronic gubbings to control the xenons, there's one on each headlight.
My only other bit of advice is be prepared to spend some money upgrading the brakes... they're awful as standard (dont know what Ford were thinking putting them on such a heavy and powerful car).
I'd definately go for a mid 2004 onwards model and with upwards of £7k you'll bag yourself a great deal, hopefully low miles on the clock.
All the best with your search... any more questions do ask.