The MK4 is a great car! My 1.8TDCi is now on 141,000miles and has been 100% reliable.

You will struggle to find full leather unless you get a Ghia and even then they are pretty rare.

Engine wise... Although there is nothing wrong with the 2.0 in both 140 and 163 format they don't seem to perform half as well as they do in the french cars.
Actually the 163 in the Peugeot 508 has a wide power band, buckets of torque and will return 55mpg+. The same engine in the Mondeo is flat as a pancake and won't do 40mpg. The 140 isn't as bad but it's still not the engine it should be!

The 1.8TDCi is a bit lazy in standard form, expecially at low revs. In the real world they won't return more than 40-45mpg (the trip computer in these MK4's are VERY optimistic). What they do have though is chunk a mid range punch! Even as standard you get 320nm's of torque (the same as the 2.0's ) at ~2200rpm and from 2000rpm to around 3000rpm they do make decent progress.
I've fettled with mine a little, I've blanked off the EGR, gutted the CAT, had a full stainless exhaust fitted and had a stage one remap and it's running 146bhp/395nms and it's not fast but it's very punchy and means you can make excellent progress without having to thrash the bullocks off it. Day to day I rarely exceed 3000rpm.
There are a few issues with the 1.8TDCi, turbo lag can be a pain in the arse but if you drive it properly and keep it above 2000rpm at all times it's never an issue. They suffer with small end bearing failure if you drive everywhere at 1500rpm, neglect service intervals and do mainly town driving. They have two timing belts, they have a belt between the crank and the fuel pump and another belt between the fuel pump and the cam. Most garages will actually tell you it's a belt and a chain but it is NOT! It is two belts and expect a bill for around £700 to have them replaced. The belt intervals are 125k/10years but I would recommend you have them done closer to 100k, there have been a few premature failures of the fuel pump timing belt.
Regular oil changes are a must in my opinion, I change mine every 6,000miles/6months. Winter I tend to use a 0w30 A5/B5 oil and if I'm due an oil change prior to summer I use a 5w40 fully synthetic A5/B5 oil. Other than that I put the recommended 5w30 Ford oil in.

The MK4 itself does tend to have a few electrical glitches from time to time. My radio freezes on a regular basis and sometimes it looses sync with the convers+ instrument cluster.
I have a rattle behind my dash that I cannot trace, it only rattles on overun around 2500rpm. My doors also rattle quite badly when listening to bassy songs of people with deep voices on the radio, something I hope to rectify with some dynamat when I have the time.
Then there is the steering rack issue which plauges the MK4. The rubber seals in the rack degrade and block the filter in the reseviour starving the pump and the rack. When this happens usually the only solution is a new rack, pump and reseviour. If caught early sometimes replacing the reseviour and flushing the fluid can give you some time but it will come back eventually.

Other than that they are a great car and I'm struggling to find anything else I like to replace mine with!