If you maintain it at the correct intervals replace any wear and tear parts that just comes with owning any car i cant see it having any reliability issues (wouldnt vow for the peugot tho). Saying that parts for a Peugeot would be alot cheaper in terms or wear and tear - suspension, brakes, sensors, bearings etc.
however you say you want a small diesel to have lower running costs but in reality how much are you going to save? £30 or 40 quid a month? is it really worth it? especially if you get a car with flat rate tax. just do the math is all im saying. my car bills arent much more compared to when i had a R reg diesel car
I think your right about the LPG, the conventional way doeesnt agree with the IS but more complicated (and expensive) systems are available.