My *guess* at engine replacement would also be oil starvation. You have to keep an eye on all alfa oil levels and charging down the motorway always uses more oil than tootling around town. CXompany car drivers are not always careful about cars that are not their own.
89K miles are not enough to worry about for most other car areas so a new engine is probably not all that bad actually as you might have got a new clutch at the same time
Check for proof that the milage is genuine though (old MOTs, sevice book milage and date stamps etc). Check that the engine is a genuine alfa replacment and if it is warrentied. Personally I think you buy a non-alfa recon engine at your own peril.
Chuck it around some bends to get an idea if the suspension is taught enough. Any odd knocking or shifts in the back end in these manuvours can be suspension bushes failing. Be fair though, at 89K miles the shocks will be getting a bit worn and bear in mind that a V6 is nose heavy.
In fact you can assume a worst case scenario (unless you have documentation to the contrary) that a number of rear bushes (there are loads on a 156) will need replacing, front upper and lower wishbones will be getting worn, brake discs will be getting worn, exaust will be getting rusty, AFM is failing. It could easily cost you £500 for some of that lot in your first year. It doesn`t sound good but its best to be aware of the worst case and money to keep in reserve.
That all assumes that the engine is good
The good side is that its all worth it
Its a terrific car to own.