Im buying a ford focus diesel td1.6 its got 98200 on the clock

Like i say..Timing belt "ask for documentation on replacement" if it hasnt been done then you need to get that done pretty much at the mileage its on / DMF noise,clutch judder when pulling off theres alot of vibration felt on the pedal and noise from the gearbox bellhousing then DMF failure is likely .so a big job expensive job to replace... Turbo leaks , essentially oil leaks on the engine - fuel leaks around the injectors seats/carbon deposits around seals need to be checked for - engine managment lights or any other warning light for that matter needs to be out...Essentially you want to be starting the car on a cold engine..and check for any unusual coloured blue or black smoke out the back when revved up..that may shows signs of turbo failure or sticking egr valve..
on test drive listen for any drivetrain noise or rumbling and ensure the gearbox does select smoothly and the clutch bite position isnt too near the top as that can show signs of clutch wear..clutch bite should fairly near the floor or before half way on the pedal before fully released... Brakes can be checked externally if it has alloys as you see the pad thickness and disc surface condition that way...Tyre tread well obviously if there is excess wear on the inner or outer tread then you maybe looking at alignment problems due to suspension wear or steering rack/balljoint issues...Check previous MOT slips for anything on advisories as that can gove a good indication of what it may need done if not aleady repaired....Ensure that EVERYTHING electrical inc central locking works in the car...might also be worth doing an HPI check on it too..there is websites you can get that done on....Does this helps further?