Bad electrics. Watch for the fuse box as it's not that well protected. Oxidising and rust can be a big problem in these cars if not cared for.
Gear box blew at around about 290,000 k's. Not bad I guess. Bloody expensive to replace though... Have spent more money on restoration than the value of the car itself (worth it)!
Don't use any other tyre than Michelin. Handling will be downgraded otherwise. In Australia, impossible to find alloy spec rims. They're all taken...
Initially, when I got this car, I never really looked after it. Only until I started to take it on long trips, did I fully appreciate the real meaning that the manufacturer intended. That is, one of the best grand tourer cars around. I've been in a lot of cars made in the 80's, but none than the 505 has more comfortable seats, ride, handling, heating etc...'than the 505' (excluding bmw's) :). Straight line speed from the 1.8 4cyl engine may be a little soft, but, it's not straight line speed these cars are intended for. Around corners, your on rails. It's very hard not to feel confident in a 505 or any pug for that matter. The 505 will keep up with the most modern hog cars around bends these days!
For a 21 year old car with power windows, best seats in the market and a good reliable engine that loves the rpm's! Get a 505. There are heaps around if you look. Just make sure it's been loved. It 'will' love you back!