Engines generally fine. Main issue areas I would be aware/check:-
Turbo hoses spliting - fairly cheap, not a major concern.
Fuel pumps packing in - difficult to know how worn (car should start imediately not require repeated cranking or use of accelerator).
Clutch & Hydralics - clutch pedal should feel light and not feel like it's turning inside out.
IRD, rear diff, and rear diff mounts - don't want clunks or bangs or harshness in drive train whilst driving (esp low speed turning, although you will feel resistance whilst doing this), check for heavily feathered rear tyres.
Water leak into boot storage compartment.
To buck the trend, although I would have a Td4 over a 1.8, the 1.8 definately is better off road (manual to manual) as it has more just of idle torque in practice and is far less likely to stall at low offroad revs IMHO. Td4 auto is your way forward as it eliminates the clutch problems and is better offroad as obsticals can be tackled more slowly and power fed in more progressively.
Mark.