Listen for a healthy turbo whistle,
not too much soot on hard throttle openings,
and at 100+k, very likely ready for top or bottom intercooler hose unless they've already been done
you don't want a dash board like a Christmas Tree with warning lights and fault codes, and the 16v is certainly more sensitive to this than the 10 and 20 valve engines in my experience,
but mechanically, unless you hear anything obvious, there should not be an awful lot to worry about at 106K, but it would be nice to see a history of Cambelt and tensioner, including water pump, possibly a clutch too would be a bonus?
apart from that, go for a good mixed drive, work the motor and turbo hard if you can, but also give the front suspension something to think about, as with all 156's, top and bottom wishbones joints, bottom ball joints etc etc can be very prone to being a bit creaky, squeaky, knock and even bang , but having a heavy diesel lump up front only exagertaes this weakness.
hope this helps, and good luck mate,
they are a good drive if nicely looked after