If the records of service history and regular maintenance all match up with old MOT certificates etc... Then the mechanical side is all but done and dusted, this can be done without having to view the car.
What I tend to do when looking at a 156/147 GT is take a trolley jack, set of ramps, some basic tools and an inspection lamp.
An honest seller regardless of the how much they are asking or age of the car won't mind you having a good look if they have nothing to hide.
Start the car from cold and listen to see if there is a diesel sound initially... It could need a variator very soon which is more cost effective to do the belts at the same time... so If it the owner says the belts have only just been done, then you know it was a quick belts only job and not the best of cam belt changes
Get the car on the ramps and inspect the floor and sills... Usually small amounts surface rust is to be expected but a good one will be well treated with under seal. Check the exhaust condition and any engine/ gearbox leaks while you are there.
Now the tools come in handy to take off carefully the trim panels and kick plates along the edge of the carpets... Inspect the inner floor pans ... Some 156's suffer from rust from the inside outwards... usually due to condensation, leaks from door seals, window seals and air con problems so it is certainly worth while checking for the tell tale signs...
If it's all clear then you know you have a good 156... Now all there is to do is take it back off the ramps and use the trolley jack on each wheel and check for play in the steering and suspension components by wiggling 9-3 and 12-6 positions... Bottom wishbones can be checked with a long bar and see the deflection of the front and rear bushes.
Spin each tyre and listen for grumbling which will confirm wheel bearing condition. A quick glance through the wheels should tell you the condition of brake pads and discs...
Take it for a test drive, Make sure it is up to 85 to 90 degrees within 6 miles or so... don't be other enthusiastic at first... Listen to the suspension components on the road surface at first... Any knocking, squeaks and groans will need attention to parts... Once warmed up check the performance... It's natural for a TS to feel lazy below 3,000 rpm but when it comes on cam, it shoulder be eager and willing to rev all the way up to the red line and the speed will be rapid in comparison to other 2.0 cars....
After that listen to the engine on tickover... It should be stable, smooth and constant, not lumpy or misfiring..
There you go... If I have missed anything , I'm sure other forum members will help out