Hi Andy, I have a 1996 155 2.0 sportpack in rosso red.
First of all dont worry about mileage. trust me. Mine has just over 146K on the clock and I got her bhp output measured a few weeks back. It was running at 148. Thats not bad considering they roll off the production line at 150!!!
Now the only bit of rust/corrosion I can see she has at the right rear brake pipe. Like jonjeffryes said the most important thing is probably the cambelt but the cam variator should be done also with the belt. If you ever start up the engine and hear what sounds like a diesel rattle for a few seconds your cam variator should be replaced.
Very important also to check is the rear seat seatbelt buckles. I have experienced on more than one 155 these not working. In Ireland thatfails our MOT equivalent. Safety fault.
Jon is right also about the wipers but you can live with it. Another thing to watch is the speedo in the binacle, mine doesnt work and this is common for them to fail so you rely on the rev counter for speed... seriously.
Given they are a low car and yes some get lowered which is stupid, you should have a good look at the undercarriage for knocks, bits fallen off, especially along the exhaust. If you want to open the boot from the cockpit the buttin is in the glove compartment and mine doesnt work so check this aswell.
Between my checks and all the rest you might be a bit put off about these cars but trust me you will NOT be sorry you bought one. Its like I say
If the whole world drove a 155 it would be a happier more exciting place
Also be patient, I had to wait a long time to get my 155. Its rosso red with sportpack and a 2.0 litre but it was worth the wait. If you dont like the purple then dont buy it, be a bit patient because there is plenty of 155s not just on autotrader but look at ebay aswell and pistonheads.