Check out
www.bmwland.co.uk for lots of info. There is a bit of a buyers guide here:
http://www.bmwland.co.uk/forums/view...p?f=4&t=108166
The touring is the E91, with the saloon the E90. Nice cars if you look after them, not exactly bullet proof as they get older, but not too frightening to maintain either. Easiest used option if you've got a good main dealer near by is to get an AUC (Approved Used Car) you get a good warranty which lasts a year and can be extended. You pay a premium for the AUC peace of mind though.
Petrol vs diesel is really your preference. Diesel does more mpg, but generally costs more in maintenance and to buy, there's not that much in it overall. Diesels often feel faster because of the torque, but are less refined and don't sound as nice as the petrols.
The 6 cylinder cars (petrol and diesel) are generally more refined and better spec'd than the 4 cylinders, they also generally cost more (maintenance, fuel, buying), but a lot of people think they are worth it. There's really a huge range of engines, from the sensible 320d to the very fast 335i, with many in between it's down to what you're looking for. The 325d/330d are a pretty good balance between economy and performance.
Diesels in particular can lead to big bills if they go wrong, hence why it is worth looking for an AUC car, you always want to see a good history.
Option lists are extensive, so if you see a feature on one car don't assume another will have it. If you like gadgets there's lots, but it can be difficult finding a car with all the ones you want.