the 110 engine is an older type of fuel delivery. more reliable but a littke less refined. i have one myself thats mapped with pd130 turbo and other bits and the thing flies, keeps with nazda rx8's and things but still returns 50ish mpg
the 130 and 150 engine is the newer type. they are slightly quieter and more refined but often suffer from a thing nicknamed chocolate camshaft syndrome. its basically a cam wear problem due to the fuel injection system. they do have a 6speed box though and are VERY mappable
milage usnt really a problem, mine has 187k but still good. they are all generally gt tdi which means better seats and body coloured bits on bumpers etc.
general problems are the door modules go nuts causing locking issues and interior lights not coming on when the doors open, sometimes the wipers go mad which is just a relay thats about really!
theres no shortage of em, but remember the a3's and seat leons are the same car